I highly recommend taking a credit/debit card and a little cash with you to the parks even if you're using the magic band for charging. We've had more than one instance where the whole "room charging" system is down when we're out at a park. One time we had just grabbed a few snacks/drinks and were just getting ready to pay with our magic band and the entire charging system was down -- you couldn't charge with a magic band, credit card, or debit card. The only option was cash. We had about $20 on us which wasn't enough to cover the items we were buying so we started to put them back but the CM told us just to keep them and that it was "on Disney". We did give her the $20 which covered most of our bill.
We do always go to the resort when we arrive and have every family member purchase something in the food court to make sure their magic bands are working as expected for charging. Last year we did that and our bands weren't working. I went to the front desk and found out that for some reason, the credit card attached to our file was an old one and it had been replaced (due to a data breach at some retailer) over a year before that. It was not the card *I* attached to our file for that trip but it was one we had used on a prior trip 2 years before that. I had made the down payment and final payment with the new card and the new card is what was showing in
MDE. The CM at the front desk completely removed the old card from our profile and replaced it with the new one. Two days later, the magic bands weren't working for charging again. Back to the front desk -- same problem. Fixed it again. Two days later, same problem. That time I talked to a manager at our resort who happened to overhear me explaining to the CM that this was the 3rd time we were fixing it. The manager looked at the file and said that for some reason when a certain update/process was happening (that happened every 48 hours) in Disney's system, it was retrieving our information from a previous stay. And guess what, it happened again on our last day. I didn't bother to have it fixed that time. But that whole situation just reinforced (for us) why it's important to carry a credit card with us all the time even if we're using the MBs for everything.