Rock Jam $130 on amazon until you know they’ll stick with it. My 10 year old has been using for two years and his instructor says it is still just fine.
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!
