Hi Greg!
What a fun Uncle you are! Is your family interested in adopting a 30-ish year old? I promise to play tour guide and to help in holding all backpacks and jackets.
Theoretically you should have no problems entering any Walt Disney World Resort theme or water park with your niece and nephew as their tickets and MagicBands will be linked to each specific child. When entering, each guest will tap their MagicBand to a turnstile touch point, as well as scan a single finger to ensure that the band matches its person. I said 'theoretically' because there is advice out there that for children this young it's helpful to have the parent scan the parent's finger in place of the excited child's digit as this helps speed the process along. However, for this specific trip each child will need to provide their own fingerprint to be able to enter a park with you.
As far as charging meals and or souvenirs to a MagicBand, the parents have the power to limit what these bands can and cannot do. Should the children's bands have access to their Disney Dining Plan entitlements, be sure to get their families four-digit PIN number from the parents to be able to complete purchasing transactions. I would recommend having the parents test using one of the children's bands to 'pay' for a meal before your day of fun just to double check. Or, another work around would be to use the parents My Disney Experience login and password on your My Disney Experience App and to
Mobile Order the children's meals. It's a few more steps than scanning a MagicBand, but it gives you complete control and leaves the kids to be kids in the Most Magical Place on Earth.
Bonus tip: Check out this awesome
Planning Guide for Preschoolers and Small Children.
~Kristen