Kitten and mama cat question

Hi folks,
I posted here a few weeks ago about my pregnant foster cat and you were all very helpful! Well, she's not pregnant any more. She had 4 gorgeous, healthy little kittens. They are almost 3 weeks old now and doing great. She is a very good, attentive mother. They are now getting more curious...creeping around outside their box every now and then.

However, the mother cat is doing something that makes me nervous. Sometimes she "plays" with a kitten...like she would play with a cat toy. She grabs the kitten with her paws and grabs it with her mouth. Sometimes she even kicks the kitten with her hind legs. It scares me so much!

Now, the kitten sometimes squeaks a little but doesn't really seem to be upset. I don't really think she hurts it. Is this perhaps the mother showing the kitten how to play? Or is she just bored!?! I feel like they wouldn't be as healthy and happy as they are if she were really hurting them. But it just churns my stomach when she does it. Any ideas? Is this normal?

Thank you!

I've actually had he same grog happen with many of my foster moms too... It still worries ne, but I don't think it's uncommon, and I don't think she's trying to hurt them- it still makes me flinch every time though I don't think you need to worry too much