Thanks guys.
They wouldn't end up pregnant as they are strictly indoors, but I just want to weigh the risks of each side...
Most likely they will be spayed however I'm going to have a talk with my vet also.
I know the risks of not spaying cats, but how high are the chances that a healthy 6 mos. old kitten and a cat around the age of 3-4 yrs will er, end up passing away during or, after a spay...?