Should I be worried about sand in the chain? This is about my daughter's (10yo) bike. It's fairly new, maybe about a year old (bought it off a relative who changed her mind). We're no active sporting family, but with this social distancing and working from home thing we've started to take long walks every day - about 2h per day or so. And my daughter rides along with her bike most of the days. So - light but regular usage.

Now, most of the days here have been nice and sunny, but last Friday we got some rain while walking. The next day, Saturday, she started to complain that her bike has become squeaky. Had to agree, the chain was squeaking.

So on Sunday I went to the nearest hardware store and bought a can of the only chain oil they had (this product). It's not specifically aimed at bicycles but judging by the many bicycle pictures on their webpage, it certainly can be used there.

My daughter didn't ride her bike on Monday, but on Tuesday we took it out again and I sprayed the chain thoroughly. Indeed, the bicycle became whisper quiet. Nice.

Then today (Wednesday) we went somewhere a little different and visited a nearby patch of a forest. Dirt paths, hills, was fun. But soon after we got out my daughter again started to complain that it's gotten harder to pedal and the chain is making noise again. This time it wasn't squeaking but... I don't know the English word for it, but it's the sound that sand makes when it's between moving parts. "Gritting"? "Grating"? "Grinding"? Not sure. Anyways, yeah, the chain and gears do look like they've got some sand and dirt stuck to them which would explain it all.

Now... I don't know what to do. I could try applying more of the spray oil, but my intuition says that it won't help much, if not actively make things worse. Reading on this site reveals that people are using some sort of "degreasers" and giving their chains thorough cleaning with a toothbrush... Which seems like way too much hassle and overkill. As a kid I've spent years riding my bike through whatever and probably lubricated the chain twice in total, so this sounds like it shouldn't be an issue. Perhaps it will just dry off or get ground off in a few days and that will be that? Or maybe I've overlubricated the chain or used the wrong substance or something and now there is indeed trouble brewing?

Any advice on what to do?

Update:

What I ended up doing was a lot simpler and will probably earn me at least a few disappointed sighs from the experts here. I remembered that there's a gas station nearby where they have free access to running water for washing your windshield/bike/whatever. And I just poured some on the chain and gears and the noise stopped and the bicycle is running nicely now. I'm keeping an ear out to see if it starts squeaking again - if it does, I'll apply a bit of oil again and this time - wipe it off!
It was the right thing to use the lube for the chain.

I do not know the particular product but in general spray lubes have the tendency to become too plentiful on the outside of the chain. This film of lube will become sticky as the solvents in the spray are dissolved.

The grinding noise from the drive train may come from sand or grit sticking to the outside of the chain. The fix is to remove it. You do not have to use the more complicated methods meant for bikes where drive train efficiency and longevity are important.

If you have a degreaser or a light oil like WD-40 at hand it helps to soften the old lube. Otherwise use warm water with dish washing detergent. Spray the degreaser or light oil on the chain. A stick is often useful to scrape off old dirt and grease from sprockets and chain ring. Use an old brush or an old rag with the soapy water to clean off old lube (and detergents). That doesn't have to be super thorough. Rinse with clear water.

When you cleaned the chain this way, apply your 'Krown' spray lube again. Spin the gears or let your child ride a few rounds on your yard (not through dirt) to work the lube into the rollers. Then wipe off excess lube from the chain. Do this last part quickly after applying, while the spray on your chain is still fluid. After a while the light oils in it will disappear and only the sticky lube will remain.

We want the lube inside the chain, between rollers, not on the outside where it attracts dirt and grit.

shortcut:

If you are in a hurry, it might be enough to spray the chain again with the 'Krown' lube you have. It will soften the grease on the chain. Then wipe it all off with a dirty rag. Wipe also the sprocket and chain ring. Be careful not to pinch your fingers!

This quick-and-dirty method will get grit into the chain rollers, and reduce their life. But for the use you describe, that will not matter.