We used to have Sonicare for Kids (toothbrush) + other Philips toothbrushes.
In our experience, all Sonicare attachments fit all Sonicare devices, regardless of whether they are for kids or not.
This also corresponds to the description according to https://www.philips.ch/c-dam... (from this pdf "Brush heads can be freely combined with all Philips Sonicare sonic toothbrushes, except CleanCare.").
However, we have almost only used Kids attachments on Sonicare for Kids devices (and the normal attachments on the normal Philips Sonicare), as the product compatibility is not stated so broadly on the products themselves (as far as I remember); but fitting on them is OK.