Hi Megan! I wouldn’t put a heating pad in the box with the mama and kittens as long as she is there with them to keep them warm. She will be having kittens soon, and birth is messy and wouldn’t want any birth fluids to get on an electric appliance with she and the kittens. She may have the kittens in the box, or may pick another spot to give birth. She sounds like a wonderful kitty and thank you so much for bringing her inside. I hope you have a litter box for her, because if she gets outside, she could have them out there somewhere and you wouldn’t know where, as cats are very good at hiding their babies. You will be able to tell, she will start leaking fluid, may start licking herself down there and you will see a little kitten start to emerge. Stay calm and soft spoken to her if you are there when she gives birth, that will help her stay calm. She needs good food right now and as she is nursing her kittens. Even kitten food is good for a queen, as they can use the extra fat content it provides. The gestation for cats is about 9 or 10 weeks, so good luck and keep us posted on her progress!