Use Solar Power Get the longest hose you can find.
Connect another hose to it to make it longer if you can.
The sun can heat the water in the hose as it travels through it on its way to the pool.
Lay the hose in the sun.
Some put large loops in the hose and lay the loops on a hot place such as a rooftop or dark-coloured pavement such as a drive-way.
Turn on the water enough to fill the hose and stop.
Put the end of the hose into the pool and wait until the hose heats up.
Then, turn the water on very slowly so fresh water replaces the heated water going into the pool.
While this is happening, cover the pool with a plastic sheet and the sun will keep the water warm, help heat up the water already in the pool, conserve the heated water already in the pool, and protect the water from dirt and leaves that might be blowing around.
Use the plastic to cover the full pool after play is over for the day.
In addition, you can use the whole thing as a teachable moment on conservation, radiation, and whatever else you can work into the lesson.
lifehacks.stackexchange.com/users/4698/Stan
Sometimes I will boil a large pot of water and add it to the water in the kiddie pool to quickly boost the temperature.
This doesn’t work for my wife though as it would be a bit heavy for her to carry all the way to the pool.
lifehacks.stackexchange.com/users/23311/Josh Withee
Depending on the layout of your home and garden, you can connect your garden hose not to the cold tap outside, but to an indoor faucet and select the appropriate temperature.
This will of course only work if the distance between the pool and the nearest faucet is smaller than the hose length, but many parents will want the kiddie pool rather close to the house, in order to supervise play and possibly get some work done in the mean time.
(Of course no responsible parent will leave very small kids to play alone.)
lifehacks.stackexchange.com/users/12132/Stephie
Filling the pool in the morning and letting it sit in the sun helps it warm up.
Works best if it’s not cloudy
and if you’re able to fill it up a few hours before using it.
lifehacks.stackexchange.com/users/23311/Josh Withee
To get warm water in the kiddie pool we have found it's the easiest to connect a hose to the hot water in our laundry room.
If your laundry area isn't close to a window or door this probably won't work.
lifehacks.stackexchange.com/users/25066/Adree
I filled the kiddie pool with water.
I took several black trash bags and laid them flat on the surface of the water.
The bags laid nice and flat and remained on the top.
With the sun shining brightly,after about a 1/2 hour I put my hand into the water.
The top portion of the waterfelt warm while water deeper in the pool felt cold.
The bag surface also felt warm.
This seems to work.
Of course, if the children are ready to get into the pool, itmight be hard to stop them and so in that case remove the plastic bags and let them jump in.
lifehacks.stackexchange.com/users/31472/user31472
I’ve got the kiddie pool out for my toddler this summer. The water from the garden hose is freezing cold though.  I’m looking for a way to easily warm up the water a bit. The warmth doesn’t need to be maintained; I think that if the water is relatively warm when the swimming commences, that will be good enough.
How can I easily get warm water in a kiddie pool?
lifehacks.stackexchange.com
<water><temperature><summer>
lifehacks.stackexchange.com/questions/18602
lifehacks.stackexchange.com/users/23311/Josh Withee
There are a few ways of warming up the water in a kiddie pool, but the simplest is probably to just fill the pool directly from the hot water tap. Alternatively, you could lay the hose in the sun to warm up and cover the pool with a plastic sheet once it has been filled. Also, if you fill the pool in the morning and let it sit in the sun to warm up. The options are to place a long hose in the sun to warm up, add a pot of boiling water, fill early to warm in the sun, fill from a hot tap or place black trash bags over the surface for a while.
There are a few ways of warming up the water in a kiddie pool, but the simplest is probably to just fill the pool directly from the hot water tap. Alternatively, you could lay the hose in the sun to warm up and cover the pool with a plastic sheet once it has been filled. Also, if you fill the pool in the morning and let it sit in the sun to warm up. You can use Solar Power. A simple solution is to boil a large pot of water and add it to the water in the kiddie pool. To get warm water in the kiddie pool the easiest way is to connect a hose to the hot water in the house. Filling the pool in the morning and letting it sit in the sun helps it warm up.  
Laying the hose in the sun or filling the pool in the morning and leaving it to warm in the sun is one suggestion. Alternatively you might connect the hose to an indoor hot water faucet. However, some people use a pot of boiling water to quickly boost the temperature of the water in the pool. There are few ways of making the pool water warmer. One way will be to simply connect the hose to the warm water supply in the house, if possible. Other ways include adding a pot of boiling water to the cold pool water or just leaving the pool water to warm up in the less. You might want to leave the hose lying in the sun as well to help heat the water a little.
Place a long hose in the sun to warm up first. Add a pot of boiling water to the pool. Fill the pool from a hot tap. Fill the pool in the morning and let the sun warm it up. Fill the pool and place black trash bags over the water surface to heat up.
You can use Solar Power. A simple solution is to boil a large pot of water and add it to the water in the kiddie pool. To get warm water in the kiddie pool the easiest way is to connect a hose to the hot water in the house. Filling the pool in the morning and letting it sit in the sun helps it warm up.  
Laying the hose in the sun for a bit might help warm the water up. You might want to just connect the hose to a warm water tap if possible. If you fill the pool in the morning, the sun will heat it up somewhat during the day. You could add a large pot of boiling water to the cold pool water for a warmer temperature.