switching existing devices to family sharing

Hi

I have an iTunes account and pay for annual icloud storage. I have an iPhone and iPad.

I have 3 young children (age 11, 9 + 5), each with their own iPod and a shared iPad. All of these have been on my apple account until now to share apps, movies etc.

I have just set each of the 3 kids up with their own apple ID via family sharing.

How do I switch each iPod from being in my name to their new apple ID?

and obviously they don't want to lose their minecraft worlds, FIFA points etc......

I'm in need of help!