I won't comment too much on this specific horse's suitability, but more on the possibility of outgrowing in both size and ability.
First is the size. A pony of this size (and even smaller) is IMO not too short at all for your wife... and definitely not your daughter at her age. Unless either one gains a very significant amount of weight over the pony's lifetime I wouldn't worry about either one being too heavy for the pony either. If your daughter does wind up being close to your height she may end up looking somewhat tall for the horse, but unless you're showing it's really not that much of a concern. I'm 5'3, and I bought my first horse when I was 15. We were looking in the 14.2-16hh size range as I wasn't too comfortable with large horses at the time and we weren't sure that I wasn't going to grow another couple of inches. Turns out I didn't, so when I start looking for another horse I'll be likely looking at 14hh and up.
Back when we were in high school a friend of mine who is very tall, long legged, and thin (5'10 or so and about 120 lbs) rode a 13.2hh pony. She was definitely not too heavy for the pony, but looked funny on her. People always gave her a hard time, but she enjoyed riding that little mare. She eventually did need to upgrade to a larger horse (a 17hh TB in her case) but still owns that pony and has been long-term leasing her out to a lesson program. The pony simply wasn't going to work out for her when she wanted to get into showing jumpers, but she was absolutely fine for playing around.
I guess the summation of all of that is that unless your daughter ends up being fairly heavy she probably won't be too big for the pony. However, she may wind up being too tall for the "ideal" pair in the show ring. Whether or not that's important to her is something I can't say, and in all honesty I'm not sure that's something you can predict when she's 8 years old.
Now, perhaps more important, is outgrowing the pony ability wise. There's a LOT that goes into that. In five years or so your small, timid little 8 year old may well have grown and matured into a bold young teenager that is looking to get really involved in competition. Will the pony (or horse, size isn't necessarily the issue here) still be the appropriate mount for her when that time comes? To some people that won't matter too much. They'll be just as happy to come out and play with their ponies as ever, while some may be looking for more than their horse can offer. Of course, that's a really hard thing to judge in an 8 year old! Which brings you to such questions as... would you be willing to sell the horse if your daughter outgrows it? It sounds like you aren't so willing in that department. What about leasing it out to an individual or lesson program? Could you afford to buy your daughter a second horse and perhaps keep the pony for your wife to ride? It's a wonderful option, but only if you have the funds to do so. Or would leasing a horse for the time being be a better option for your daughter to get more involved in horses while waiting to see what she really wants/needs in a horse? Lots of things to consider...