Rough play??

I have a mommy cat and a 7 wk kitten. They play a lot- but sometimes it seems that the mother is over aggressive - sometimes it looks like shes biting the kitten. Sometimes it looks like shes pushing her around. Is this normal?
At what point should I intervene?

This is perfectly normal. It's important that you don't intervene as it's the mumcat's way of teaching her kitten and strengthening her muscles. At 7 weeks, the kitten is in the middle of weaning too so mumcat may be pushing the kitten away sometimes when she tries to nurse. You may hear the occassional 'yelp' from the kitten but unless the kitten is continually yelping or mewing, she should be fine.