I am not finding a tips and tricks guide online. The new Katy and Bob game plays very similarly to this first game, so any advice you find on the new game pretty much applies to this game too.
As an example:
Chaining is key in order to get the goal score or expert score. Wait until 5 customers arrive, give them the menus, serve them all their food, then collect all their money. If you have enough money, purchase and upgrade the fireworks. This will boost your earnings quite a bit.