Don't worry too much about exact measurements. Whatever your child can manage will be fine for this recipe. Use a big bowl for whisking. You can hold the bowl steady while your young cook whisks away. You can trade places if somebody gets bored or tired.
Bananas are safe for even the youngest child to cut.
Use a serrated dinner knife or a plastic picnic knife, and put a piece of tape on the handle so your child remembers which end to hold. Add yogurt, honey, and lemon juice. You can add other kinds of fruit as well. Slices of kiwi are especially pretty. If your child wants to be involved in the advance preparations, scrubbing the carrots is an appropriate task.