RE: Phantom 301 cm or Start M?
The Start (especially the older Starts) are the best boards for absolute beginners.
Whether to get the Start L or the Start M will depend on the weight of your kids.
Once they've sailed the Start and have their basic skills( uphauling, getting the basic position "T" alignment, stepping back to get both feet behind the mast foot, balancing the rig over the mast foot, and rotating the shoulders (from the hips) to power up the sail, basic tacks and flare or rope jibes) the Phantom 301 will be fine.
Since they are kids, they will figure things out pretty quickly, and they might do all right on the Phantom, right from the start, but it would be better to have someone else in your club (perhaps someone who has a Start or can use the club's Starts) teach them the basics and then they will be ready for the Phantom 301.
Oh, and a 5.5 m2 rig is far too big for beginners. Might not be too big for you larger children, but for sure too heavy.
I use the Phantom 301 with a 4.2 or 5.0 m2 Sailworks Retor Ripper 3 as my "tether board" in our "A Taste of Windsurfing" Program,and I put the students on either a Rio M (little folks heavier than 80 lbs.) or a 2001/2002 Start for the heavier students. The little one's use a 2.5 m2 Retro Ripper 3 and the larger kids get a 3.3 or 4.2 m2 Ripper 3 depending on the wind conditions.
For smaller kids, I ride on the front of the Start with them as I don't think it's a good idea to send peoples small children out on a board all by themselves. They can have problems and get so frustrated they "give up" on windsurfing. If they can handle it well, they get a board with a small sail on the tether, right away, but they have to show me they can uphaul and master the other basic skills first.
Hope this helps,