Howdy,
The short answer is no, the .410 is not legal for SASS.
I don't know how old your step-sons are, but there is some lee-way under 12. Also, check with your local club and see what they will allow as a temporary use shotgun. Local clubs can be very accomodating.
I'll also suggest this: The 12 gauge with a properly fitted stock and "featherlight" ammo has less felt recoil than any 20 gauge. My son, now 12, began shooting CAS about three years ago and never liked the shotgun, he had his own 20 but it kicked. I finally got him to try out a smaller friend's 12. He's convinced, now he only shoots 12 gauge.
Good luck to y'all.
Adios,