Cut the tops off of the muffins so they have a flat surface on top and bottom.
Then, add layer of frosting or yogurt in the center.
Pipe or spread more frosting or yogurt on top and around the sides if desired.
Decorate with sprinkles, fresh fruit, or dried fruit crushed up.
Notes
Store-bought or homemade muffins can work great for this! You can also choose something gluten-free, nut-free, dairy-free or anything else your child needs. Frosting, filling and decoration ideas:
Store-bought or homemade frostings (any flavor)
Thick Greek yogurt makes a great frosting substitute if you want less sugar
Fillings could include apple sauce or baby food purees they love
Whipped topping (Cool Whip) is a light/less dense option
Sprinkles or other small (no choking hazards) edible cake decorations
Fresh fruit like cut strawberries or blueberries for the top
Dried (dehydrated) fruit crushed up makes a colorful and sugar-free sprinkle substitute