With a nearly three year old son, you are perfect for a BLT stay. It would be my choice, too. Yes, the GF is wonderful as well, though.
Walking from BLT or the CR to the MK is simply AWESOME!!! If possible, I would switch to BLT in your situation, and get a two bedroom on rented points. You can get a lock off so that your parents have their own space, but you can all still share the same living space. You will have a full kitchen and ,living room, three bathrooms (IIRC) and a laundry right there in your room. You will have a king bed in one room and a queen with a pull out bed in the other bedroom. If you opt for a 2 bedroom dedicated villa, then you will have two queens in one bedroom and a king bed in the other. I'm not sure how much you are paying for two rooms at the GF, but you might find that a points rental for a 2 bedroom at BLT would be comparable. Worth checking out, at least to see how much and, if available.
One thing to be aware of is that you cannot change or cancel a rented points reservation. Plus you need to b book at least 7 months in advance to get this.
You are sometimes able to get cash reservations for these through WDW, even on discounts, so you 'might' be able to get this through your BB offer. I don't know.
Either way, staying in a MK resort is your best choice for your family. two rooms at the CR or a two bedroom at BLT would be the best choices, but GF would be a very close second choice.
Enjoy!