I fostered a mother cat and her 5 kittens from when the kittens were 3 weeks old to 8 weeks old. All but one of the kittens had moved to dry food by about 5 weeks, but they all still suckled mum. Generally after they'd been playing, if mum was nearby they'd nurse until they fell asleep, and they wouldn't suck for long, they just liked lying with mum and kneading her belly.
The one little guy who wasn't really eating kitten food fed from mum regularly, and even at 8 weeks when they all got spayed and neutered (invluding mum) and were up for adoption, he'd be feeding from mum. She's started pushing him away and he was eating kitten food by this stage, but was a little guts so got in a feed whenever he could
Luckily he was amongst the last of the kittens to be adopted, and went to a home with one of his brothers, so I don't think he was too traumatised by the whole experience.
I was so worried the mother (Cleo) was going to be upset about her babies being taken away (I would be!), because she was such a fantastic mum - very attentive and just so good with them. My neighbour adopted Cleo, and she is a completely different cat now. She's only 11 months old or so, and now she plays and plays and plays, and loves cuddles, is a lap cat, and is soooooo happy!
So I would recommend that you don't worry, just let it be