You can start giving the kitten milk (not cows milk) in a bowl now so they know that not all food needs to come from Mum - they will soon catch on.
They will naturally start to show an interest in Mums food when they are ready, make sure her food is kept where the kittens can reach it, I would advice a good quality dry Kitten food such as Hills Science Plan - that's what cats protection supplied us with when we were fostering, it is expensive, but wet food will go off if left out and food needs to be freely available for them 'ad-lib'.
If you monitor the kittens weight a couple of times a week you will spot any problems quickly - we had one that wouldn't take solids and didn't gain weight as well as the others and it turned out he had an umbilical hernia, (these are quite common and are fixed at the time of neutering) we encouraged him to eat by soaking cooked coley in kitten milk.
When they start eating solids they will need a litter tray with non clumping litter too as mum won't be able to carry on keeping them clean indefinately.
Hope that helps.
Lorna x