Another few tips:
Start feeding one character until it reaches level 15.
This is very time-consuming, but I don't think they let you know that goal until much later.
A strange tip about XP & Carts.
Repeatedly buying, upgrading and selling foodstands can help nudge the XP bar to the next level. It seems pointless, but if you're stuck on a level without much to do, this will help push you forward. I'm quite far along, and that's why.
Keep generating characters on each stage even after you have all of them.
It can help provide some much needed money. If you place and then sell a generated character - even if you don't need it.
Also, take heart if it's taking you a while to get characters. The failure to achieve the right character the first few tries is a necessary part of the game because it builds your XP and gives you cash.