If space is no object I would recommend a grand piano or a full sized upright. Kids can tell the difference in instrument quality, and a large soundboard is very encouraging and may keep them interested in playing. If that is not an option, then a good digital keyboard with full sized keys and a responsive action is a reasonable choice.
If the kids are interested let them try out various instruments and participate in the decision; they may be more invested in the instrument and practice more!
This became a pandemic purchase. We bought a keyboard out of convenience, and because we didn’t see piano lessons happening anytime soon, needed something with headphones, and didn’t want to spend a lot. I chose the Casio CTK-2550 PPK 61-Key Premium Keyboard and have generally been happy with it’s feel (for a keyboard) and versatility for total beginners. We were using iPad apps for “lessons” but I had limited patience for integration with the keyboard, and didn’t find the Casio app to be useful at all (but again, this was March and April, so that should be considered).