You say the kittens are just 6 weeks old. If it were me I would wait a while, at least till mum decides its time kits started to stop suckling.
With her babies only 6 weeks she will want to get to them if she is kept away.
They are all different of course, some mums seem content to suckle their kits for ever,lol but on the whole, when they are a few weeks older, she will start to push them away.
If you get her spayed then it will not upset her so much, and kits will be a lot more independant.
Hope this has helped
Obviously keep her inside in the mean time incase she starts calling again, or she will be pregnant before you can get the op. done.