Do it. I bought both my 11 and 9 year old apple watches when they each turned 8 years old. Don't regret it one bit. Being able to communicate with them over text helps you see a different side of their personality. I also control their contacts, apps, and being able to contact them whenever I want and see their location and also get pinged if they leave a spot has been helpful. Its also helped them be more responsible since they use it for alarms, reminders, and they've learned a lot just asking siri questions.
If you buy one, spend the extra and get a series 9 since the battery life is way better than the 8 and older (new chip, bigger battery). I just upgraded my 11yr old boys from his original SE that came with cellular. They also have a typing keyboard which is helpful. My 9yr old has an SE 2 which shes had for close to 2 years.
Way less expensive than a phone and they aren't on them all the time like with a phone or tablet. Cellular costs $10 a month and Applecare is cheaper.