We've stayed at Grande Vista a few times and have visited at Harbour Lake a couple times but not stayed there ourselves. I don't own at either of those Marriotts nor am I a Marriott timeshare owner so I have no particular allegience other than I admittedly like the Marriott timeshare resorts a lot and generally recommend them.
So, with all that as disclosure, I'd suggest that both of those resorts have their good points and would provide a very good vacation experience.
Harbour Lake is the newer of the two - meaning that the original build occurred more recently - but Grande Vista has been undergoing a complete refurishment to bring the villas up to current Marriott standard. The edge there goes to Grande Vista as even in their newest state the villas (suites) at Harbour Lake were never intended to be at the top of the Marriott scale of timeshare units. (Harbour Lake was originally built as a Horizons by Marriott brand which had been sort of an entry-level timeshare option that was slightly below the quality of the full-blown Marriott resorts. I believe that the few Horizons resorts have since been absorbed into the Marriott brand but the higher level of quality hasn't been matched as of yet.). Now that probably means more to people who have stayed at one, or more, of the higher quality Marriotts than it would to someone who would be staying at Harbour Lake as their first Marriott resort experience as I think that Harbour Lake is a nice resort.
The great feature at Harbour Lake is the pirate ship water feature that's aimed at younger children - it's really pretty impressive and has created a new standard in the Orlando area and forced other timeshare resorts to develop similar interactive water features (a win for timeshare owners and guests with small children!). There's also another pool at the resort.
Grande Vista is, indeed, much larger than Harbour Lake and features two large pool complexes along with two smaller pools that serve guests who are staying in building that are further away from the center of the resort and who don't want or need to make the trek to the big pools. There's a resort store and there are food options near either of the two main pool areas. There's also a traditional sit-down restaurant on property at the golf clubhouse.
The Grande Vista property is a bit more private than the Harbour Lake property but both are well situated for relatively easy access to any of the Orlando-area theme parks or other attractions.
Which is best is certainly a matter of your particular needs. We actually prefer a third Orlando Marriott resort - Cypress Harbour, located across the street from the Harbour Lake resort - but if our choice were between GV and HL (and we've had just that choice a couple of times) we'd pick Grande Vista because we prefer the larger and generally nicer villas. Of course our family situation is different from your's as our youngest child is about to age out of her teens and long past the need for the cool water park-like feature.
I think your family would have a great time at either resort.
Whichever you choose - Enjoy!
Dick Taylor