You don't need any skill and only a little time to customise a birthday cake with sprinkles. You'll have to forgive me for sharing something so basic but it's a great one to have up your sleeve.
: 15 minutes
: Any Age
Smother a cake in butter cream icing.
Cut out a number from a piece of paper (the age if the child).
Lay the number on the cake.
Liberally cover with sprinkles which is also a good step to include kids.
Peel off the number template.