entry level board for kids

[Gades]

Hi there,
I'd like to buy some boards for kids between 7 and 14 years old, for beginner's course.
At the moment we've got some Start M and L (just for big adults). Most kids find the Start M quite easy and stable, but when they try to turn / tack is not that easy. They're using most of the time 3 - 3.5 m, and the wind is up to F3.

Should I just change the fins for smaller ones (the original ones are huge)? or should I get smaller boards, if so, do I go for the Start S or the Starsurfer S / M?

Thanks for your help!