Swabbing a child for dna?

if i swab a child for dna but don't imediately mail off results will I always have the opportunity to turn that swab in to get the results. Im asking because its costly and I may not have the all the money so I want to be sure that the dna will always be available on the swab. Plus the child will not be in my custody long enough to take care of it at a later date