砂をはらって！って英語でなんて言うの？

Any of these phrases should be adequate.

Just tell him/her to "clean that sand off (your legs, arms, or whatever body part)"

Also "Dust" works instead of clean.

"Darling ,be a good boy /girl and brush off that sand."
By saying "Darling" you are encouraging them to complete the task without much trouble.Darling is a word we use to speak to our loved ones.

"You need to brush off that sand before you get into the house/car."
This is to issue an instruction that needs to be followed without argument or questioning.

Children are notoriously messy;-))
Sweeping/ Wiping /Dusting, Cleaning /Clearing off....and so on;
These are all valid expressions, mainly dependent on the source of the dirt;-)

When you want to instruct your child to remove send from themselves, you can say:
-Remove the sand off yourself
-Brush the sand off your body