Yes, it is legal — and smart. It saves the business the cost of printing paper checks, lets them be sure the money is appropriately deposited, and that there will be no undeposited outstanding checks to track. This is the perfect time to set up a free checking account (as your bank if they have free checking for minors and elderly) and teach her how to guard her credit online, build a credit history, and use money wisely. I’d love to give her a copy of The Savage Truth on Money because the first third of the book is aimed directly at her!! (You can get it on Amazon!) This is a gift of an opportunity!
(And don’t chose the debit card alternative as it is nothing but an incentive to spend!)