Consider this: iPad mini - Apple — mini has a bit more display room than does an iPhone, has good cameras, and it supports Pencil. And Wi-Fi and cellular models are available, and cellular models only need a cellular plan if you want to use that feature.
Or as mentioned above, an iPhone 14 or other recent model with no active cellular plan—you'll need a SIM or eSIM to initially activate, but a cellular plan is not needed for any subsequent Wi-Fi-only use.
Given the bumps and drops that pretty much every little person encounters, particularly those little ones seemingly likely to engage in parkour or other super-amazing danger magnet videography techniques, also budget for a robust case, and for AppleCare+ and/or for potential wholesale replacement when (not if) it gets dinged or dropped or dunked or lost or stolen. Budget for device backups too, because budding videographers need lots of storage, and probably aren't the best at backing up their work.