For me, one potential drawback for putting the money on a disney debit card would be if you had to cancel your trip for some reason (medical emergency, death in the family, hurricane). When I got closer to my trip in February this year, my father was very ill, and I didn't know if I'd be able to go (or if I might be called home suddenly).
Lots of folks seem to put all of their spending money on Disney gift cards. We're talking hundreds and hundreds of dollars! You just never know, something unexpected might come up. I'd sure hate to have my money tied up!
But OP, I think if it's a small amount of spending money for your kids, you'd probably be fine.