I picked up a young pregnant stray about a month ago. She had 3 beautiful kittens 2 weeks ago and everything went well. About 3 days ago she went into heat again. One of the kittens is really adventurous and will get up and try to walk around. A few times now I've seen this sequence of events: Mama Nina will sit outside their kitten area and watch them, cooing a bit. Little Guy gets up and wobbles on over to her (their box only has a short wall on one side). When he gets to her, Mama starts licking him hard & then playing rough, biting his neck & kicking at him with her back legs & he cries out. I've heard cats do this either to punish their kittens or to teach them how to hunt. But he seems way too young at 2 weeks.
Does this have something to do with her being in heat, or does she think the little guy is ready to play because he made it out of his box? Is there anything I should do (make it harder for kittens to escape maybe?), or should I trust Mama Nina to do the right thing?