Is this a smart idea?

For Roblox and Minecraft, a TV should be perfectly fine and in fact excellent. I will go out on a limb here and say that even for most 'real' games a TV is fine. The latency associated with TVs is most noticeable in FPS games. For other genres like strategy, third-person adventure games etc, I do not think it matters as much if at all. Many people, especially those who have not used a low response / gaming monitor, do not even notice a lag at all (Note: You will find many such people in real life but never ever on the internet). It would be nice of course if your TV had a "Game Mode" which lowers latency, but it may not necessarily be there in a 10-year-old TV (though it was not that uncommon even back then, so do look for it in your TV settings).

Regarding programming on the TV, I think the situation is slightly different. Using small text in general doesn't work for me at all on a TV. Most TVs, other than OLEDs or recent non-OLED ones, don't seem to handle text well enough in my experience. There's either ghosting or some other manner of artifacts which makes the text harder to read compared to a monitor (apart from the distance from TV involved). I commonly see this issue even with office televisions used for mirroring laptop output. Maybe playing around with sharpening and other settings might get it to work well enough though and it really depends on the specific TV in question.

Overall, I feel you should be fine, at least for gaming, but probably for programming as well. I have a couple of gaming rigs hooked up to my living room and bedroom TV's and I quite enjoy gaming on them. The much larger screens and ability to lounge about while gaming more than make up for any perceived or actual lag for me.

I hope your kid and you have a great time with your new setup. Have fun! :)