are miniature donkeyts rideable for kids?

It is possible to "ride" anything.
Donkeys are a little tricky to get broke, horses and mules are easier.
General rule, the horse should only be asked to carry 1.5-2% of their body weight.
Although, they can pull up to 5%, and conformation can make those #'s flucuate some.

Yes they are! And never a dull moment either. I see them sometimes at state/county fairs with the children's pony rides. I see Zebras used for the pony rides also a handful of times.