The only real difference is the pinpointing vs. non-pinpointing, and some extra programs on the ACE 250. To decrease the frustration associated with learning a metal detector, the ACE 250 might be the better choice. However, should you just use the sniper coil, which would really make it easy to learn the machine, the ACE 150 would be sufficient. Because the sniper coil is so small, it is very easy to pinpoint a target just make X's over it. Another option would be to get one of each, one with the stock size coil, one with the sniper coil.
Either way, I bet they have a lot of fun. And....if you post their finds, I bet they will be really smiling when they see all the comments they get for their finds.