First of all, you have a wonderful style already from the examples you sent - very European and slightly quirky! I really like it, especially the soft colour that you use, and I take it you use traditional media?
As for drawing children, it's probably best to start with videos on youtube on how to draw humans, rather than just children as it's probably beneficial to be able to draw people of all ages just incase you ever need to. Do you know of any life drawing classes happening near you? Life drawing taught me how to draw the human body.
If you develop a style that's not completely anatomically correct in the future, then it will still teach you how the body bends and moves so that you can illustrate gesture really well with your characters.
When I draw a character, say its a child for example, I start by using shapes. So a circle for the head, oval for the body and usually lines for the arms and legs and then ovals for the hands and feet. This helps me to get the pose right... (an example i found online) -
I wouldnt run before you can walk though, start by learning how to draw the body as a whole and when you're confident enough then move onto hands/feet etc, then start trying different ways to capture emotion on your characters face.
it all just takes time and practice just use whatever images you can online, magazines, real life, photos etc and just draw lots and lots until it clicks which it will!
Your style for drawing people will then naturally fall into place to compliment your lovely environment illustrations!
All artists need to use reference material, so don't worry about doing that - the more the better in my opinion, there's nothing wrong with it.
I think all illustrators/artists have that horrid crushed feeling after finishing a piece that hasn't turned out the way they hoped, that's completely normal and very good for making yourself improve and getting that piece to the level you want it to be, but it just takes practice Don't feel down, at least you're drawing and TRYING! That's brilliant and such a great start and you WILL get there because you seem like you have the determination.
This forum is brilliant for feedback, so if you have any examples of your drawings of kids then it would be lovely to see.
All the best to you with it