When your little one is ready to transition into a bigger bed, it's time to look at kids mattresses. Kids mattresses can range in size from twin, twin XL, to full, depending on how long you want to keep the mattress and big your child gets. You want to make sure they have something that they can grow into, but not so big that they can't easily get out of bed.
It's usually best for children to sleep on a medium to firm mattress, but once they're past the toddler stage, it's best to go with whatever they feel the most comfortable sleeping on. Have your child try out different mattresses to see which feels best to them.