If you are really worried that the kittens are not gaining weight (average about say 10 g each day) or they are not feeding etc THEN take them to the vet. If they are feeding and growing and mum cat is eating well, no need to take them to the vet.
At the age of 9 weeks when the kittens have their first jabs, then the vet will give them a thorough health examination. You can make the appointment to spay the queen then if she is doing well.
Hope that helps a bit ...