Sorry to be coming in on this one late, but I've heard others talk about it not being a huge problem to take someone extra into an HGVC timeshare unit with you (especially if it's just a baby, I would think). It sounds like the important thing is really just to be discreet.
By the fire codes and whatever else, the units are limited to the number of occupants advertised. That said, I suspect as long as you're not being ridiculous and making a ton of noise in the process (like trying to pack a dozen partying college kids into a unit built for four), it probably won't be a big deal.
There are two things I would suggest doing to keep it on the low down if you're going to try the studio with the three of you (with one being your 1.5 year old): (1) Just one of you check in at the front desk, with the other off somewhere "parking the car" or whatever, so there isn't even any attention brought to there being a third person (albeit a baby) with you, and (2) bring a pack and play or port-a-crib with you (an easy piece of check luggage), again keeping the issue away from the staff as far as possible.
I'm certainly not trying to encourage anything that might cause you trouble, but I've just heard enough people talk about it, both in person and on this board, that I'm reasonably convinced it's at least worth considering.
Good luck, and let us know how it works out.
