Vomitting can be a sign of many things. Has Boots been wormed recently? She and her kittens both are at an age, where worming for parasites is a good idea, and keeping up with a good worming schedule. Please remember to get the worming medication from your vet, at the recommended dosage, and not get OTC wormers, which can make your cat/kittens sick.
A call to the vet is in order, hope Boots is feeling better soon.