Firstly, as an investment, watches are shit, the vast majority will lose value.

Secondly, as a gift idea this is also shit. You’ve no idea what sort of thing your kid will like when you finally hand it over. They might not give any fucks at all about watches or they might only care for a particular style or whatever. The chances of them actually liking the watch you choose are pretty slim.

Thirdly, what’s the actual plan? Buy the watch, stick it in a drawer, wait 18 yrs and hand it over? Cool, what about maintenance etc - I’m not convinced an automatic which has sat unused for that long is going to work that well. So now you need to spend money on getting it serviced. Obviously you could buy quartz and remember to remove the battery so it doesn’t corrode over the inside but now you’re gambling on unused electronics maintaining their function after sitting unused for a significant period? Either way that’s a gamble.

Obviously you could wear it yourself but that’s just buying yourself a watch with extra steps so not ideal.

Finally, the idea of trying to force some faux sentiment into an inanimate object to consciously create an “heirloom” is weird in itself, regardless of the value of the watch.

If you’re a complete prick of a parent (and I’m not saying you are/aren’t) but leave them a watch which has somehow appreciated in value they won’t give a shit about the sentimentality, they’ll just say “my prick of a dad (assumption) left me this, I’m gonna sell it and do something useful with the money”.

Likewise, if you turn out to be the best parent ever then you could leave them a battered old Timex or something and it’ll be the associations with the watch they value.

TLDR - it’s a shit idea, YOLO into bitcoin or something instead.