One bit of "magic" can actually be very useful for making your child pick something that you want them to pick.
Say you have 5 choices. Boxes, snacks, envelopes, whatever.
Imagine they're lettered A, B, C, D, E.
You want them to pick B.
You say to them "Pick 2". They choose A and C. You remove A and C.
They have B, D and E left. You say again "Pick 2".
They pick B and E. This time you remove D, leaving the two they picked.
Left with a choice between B and E, you say pick one. If they pick B, you give them B. If they pick E, you remove E and give them B.
This actually works on adults too, the first once or twice you do it. How long it continues to function for your kids depends on how smart they are and how smoothly you remove choices, but if they're young you should get a good few rounds of them "Magically" choosing what you wanted them to have all along.
It works best if they aren't fixated on a specific object right from the get-go, so you might need to anonymise it. For example if they are demanding a chocolate bar, and you want them to have fruit, then write "Chocolate" on 4 pieces of paper and "Fruit" on one piece of paper, but make sure they don't know which one says "Fruit" - but you do. That way, when they complain about receiving fruit, you can quite truthfully point out that "Chocolate" was written 4 times, so they must have just been unlucky in what they chose...