We usually mix kitten food well mashed with a little cat milk, 10 seconds in the microwave and volia kitten gloop, they will walk through it and wear it and it is messy but they grow to love it
If you can get your hands on some RC baby kitten biscuits for baby kittens then thay are good to very tiny and not a lot to crunch but gets them used to the idea
Also what are you feeding Mum, our Mums have always been fed kitten food as they need the calories so try doing some for Mum all mashed up and let the kittens loose usually they get the idea if they see Mum eating real food but most kittens will still like Mums milk bar till they are at least 8 weeks old, usually our Darling Harvey ends up with kittens sucklling on him he loves kittens