I would generally only recommend a sonic toothbrush, even for children, as the good brushing result is better achieved with it. This one would be suitable for children.

Philips For Kids

With rotating or oscillating brush heads you can also do a lot of things wrong, because they need a special brushing technique and not everyone can do that, which can lead to damage to teeth and gums if done incorrectly.

Yes, the Oral B attachments fit both the Kidz and the adult appliances. I have just tried it out myself on my daughter's appliance - no problem.

No, they are not compatible.