Question: I have a 4 year old daughter and a 2 year old son. Both are good kids and just have the usual kid moments here and there - but on the whole they're very good :)

However, whenever we go out anywhere, literally anywhere, they always pine and try to get a new toy or little trinket... The scale really doesn't matter, they're happy with a 50p or £1 toy, or squishy thing or whatever it may be. For me, the cost isn't really an issue - let's face it, we pay significantly more for a tepid cup of brown liquid someone has deemed to call coffee!

My concern is longer term, will this become a problem - the expectation is set and that carries into adult life, then they'll shop themselves into terrible debt for example. Seems extreme, but I know of a number of people who are maxed out on credit cards simply buying handbags!

Answer: 
I'd say if you have a concern, you should listen to your inner voice.

I personally think you are setting up your kids to expect certain things at certain times. We all do it it different ways -- it is up to you to decide. They won't always get a treat, but they often will, perhaps only if you stop for coffee.

In my own opinion, I think a treat every once in awhile is fine and an allowance for the rest of the time works well. It is never too early to teach kids about money and responsibility. You have to gear the lesson to their understanding of course. If you give your child a dollar per week or a pound every two to three weeks, the amount is up to you. Help them spend it (and yes at this age you get to say 'yes' or 'no'), or to save it, or part of it for a bigger treat. "If you save your dollar, you may have enough for the thing you want."

And slightly off topic -- allowance is what your child wants to have for things they want, but do not need. It should not be tied to chores or behaviour, though it might be for truly egregious behaviour. Chores are what all members of a family do because that is what families do to make it work. As a child gets older allowance can include money for extras like a movie or a special pair of shoes -- but they may have to save for those items.
Kids want toys, finite. That is normal and they can have already tons, it doesn't matter. Also just for the future, expect to throw masses of these toys away later. If they get older and don't care any more about them.

However now the smartest things you could do is using that to teach them planning and decision making. For example if you know they want multiple things, tell them that they each can have one. Also introduce them to the idea that you can't always get something, and tell them if they got already many things lately, that there is no more budged for new things.

If you have the feeling that they are understanding these concepts you can go further with things like pocket money, but I would say that they both will take a view years till they get that far.