My 15m old toddler has had a scab on her leg for 2 weeks now. She scratches it and has peeled it off. I'm worried its not healing. Should I be worried?

Probably not. It's not healing because she's picking off the scab. Cover it with a light bandage, or a bandaid that she can't remove. Change it every day or two. Keep it up until it heals.
Yes. I would make sure this is not infected if your child is picking it. There are great topical antibiotics that help and you may have to figure a way to do occlusive therapy where the child can not get to it.