I suggest you start with something like a pair of 385 size motors.
They are 5 pole so run nice and smooth, especially when starting, and are efficient so not too current hungry. If you still think she's too fast for the grandkiddies you could always use the 'End Point' setting on the TX to limit the throttle travel, e.g. 75-80%, or whatever seems reasonable to you. You could then reset it to 100% when you wanted to unleash the 'full fury' and have some fun😉
Check out MFA or Cornwall Model Boats for the motors.
For the ESC an Mtroniks Viper Marine 10 or 15 would do nicely. I have a few Marine 10s, never had any trouble with 'em.
Oh, and don't forget the Motor (spark) suppression kit😉
I have a 1/32 PTB made of wood, much heavier than a plastic version I guess. When I bought it it was fitted with two 2200Kv brushless. The chap I bought it from said he was selling cos it was much too quick for him! We'll see. It's currently under refit (like many of my boats🤔).
Hope this helps🤞
Cheers, Doug 😎