Key weaknesses of Pico-8 for kids learning to program:
1. The editor runs at the same resolution that the games do. Maybe this is because they didn't want to make their VM flexible enough to deal with multiple resolutions, or they just wanted to make the illusion that the Pico-8 is its own machine very consistent, but the end result is still a painful code editing experience. When I make example programs for my kids, I need to compromise between descriptive variable names and indenting code blocks.
2. Deviations from the standard lua API/libraries mean that normal learning resources like PIL have to be interpreted with an asterisk.
3. This is a minor thing, but the fixed palette is annoying. In graphical terms, Pico-8 lies roughly between the Atari 2600, 7800, and NES, but all of those systems handled colors more flexibly.
4. Splore allows children to find games that are not appropriate for their age. It's just not designed for kids and parents. My son found a game featuring smoking and thought it looked SO COOL (this was easy to disabuse him of). He also found a game that puts you in the role of a cyberbully, and the subtle psychological effects of that sort of "edgy" game aren't as easily undone with a single conversation. It is possible to mitigate this by putting whitelisted carts into the bbs carts folder and then making that folder read-only.
TIC-80 improves on the first three points. The editor is still running at the same resolution as the games, but uses a higher resolution (and the editor can optionally use a thinner font). The lua implementation is much more standard. And the palette can be changed (even per-scanline, I think). The music editor lets you draw your own waveforms, but in other respects is less intuitive than Pico-8's. (I haven't looked at TIC-80's cart browser so I can't comment on it.)