My child a little bumps all over legs?

Start with a photo. The majority of childrens rashes are viral. A quick visit to your child's doctor could certainly prevent a lot of worry. Rashes with fevers should be seen. Red or purple rashes that don't blanch (go away with pressure), are more worrisome and deserve an exam urgently.
Hard to say. Not knowing your child's age, could be dry skin, bug bites, eczema or a viral illness needs to be seen.