The problem with training kids is that few of them are very disciplined.
You can coach most 11-14 year old kids into decent form, depending on their level of coordination and your coaching skills, if you catch them in a moment of enthusiasm.
And they can put on muscle, though not like an older teen.
But it's a very unusual 12-year-old who will lift consistently as a volunteer.
If you have such a kid, that's awesome.
But if your kids are as flaky as mine have all been at that age, you'd have to be pretty harsh on them to get them to follow SSLP for more than a week or two. Bordering on abusive.