Assuming you have the budget for it a worst case scenario of "I made my son happy while achieving nothing -else-" doesn't strike me as terrible at all.
It costs money and there's no guarantee it will actually make him happy. It could lead to him playing the game in some dark corner where no one can find him. There are advantages to him having to use the desktop PC.
Remind me of my laptop that I bought with 2 SSDs. Not from HP or Dell, but still, I now wonder if I should replace one of them with a more recent SSD and give the other to my son (he currently has an anemic SSD that's too small to install Genshin Impact on).