All reactable applications are safe for children use. Depending on the age, however, there's some concepts that may not be completely intuitive to children of a young age.
The reactable Artist editions play tracks of the corresponding artists without needing further interaction although they can be interacted with. It can stimulate creativity and understanding of musical concepts.
My final opinion would be that it is good for children, but definitely reading/writing abilities are required to fully understand what's going on.