I’ve recently started doing this (having swapped from a dedicated “kids” watch from KidsOclock which stopped working).
It is in some ways better and others worse than the dedicated kids watch we previously had.
Better: the ability to message (iMessage) and call, and the range of apps etc. The OS is obviously more robust and the device will likely last longer.
Worse: where I live (Australia) at least, you can’t use one of the major Telcos you have set up for you iPhone and add a kids watch as a watch - you need to sign up for a dedicated separate service (from Better Roaming) and add that to the watch. It seems ok, but people complain about the network it roams on to (I think Vodafone). It is also annoying that the watch really wants to be an accessory to a particular iPhone, meaning if the kid does not have a phone, various apps won’t work (eg Spotify says that you can’t install the watch app without having it on an associated iPhone).
Overall I think it is good, but not great, so far