Want to know the responsible answer to "mommy can I buy a Fortnite skin"? It's "no". Simple as that. Same as it was when the question was "dad can I buy this Harry Potter poster" or "mom can I get this GI Joe" or "grandma can I buy a stick to roll my hoop with" or anything marketed to kids over the past literally forever.
Don't want your kids spending money on video games? Don't give them your credit card number. Don't blame Epic for making a fun and free-to-play video game that kids enjoy. The people actually spending the money aren't kids being taken advantage of, they're actual consenting adults. Kids don't have credit cards so they cannot make digital purchases.