Hi Allison,
When traveling with young children, staying at a resort with direct access to the monorail is a great idea.
Bay Lake Tower at Disney's Contemporary Resort is a lovely resort. It is the only resort from which you can walk to the Magic Kingdom. The walk is about five to ten minutes. The monorail runs right through Disney's Contemporary Resort, which is connected to Bay Lake Tower by a skybridge. The walk to get to the monorail is fairly insignificant, maybe five minutes from Bay Lake Tower. The theme at Bay Lake Tower at Disney's Contemporary Resort is sleek and modern, so some find it not as exciting for young children; however, there are still wonderful amenities such as a pool with a waterslide.
You also mention that you are looking at Disney's Grand Floridian Resort. It is also accessible to Magic Kingdom by monorail; however, you cannot walk from there to Magic Kingdom. Disney's Grand Floridian Resort & Spa also has what some consider a slightly more adult theme, that of Victorian elegance; however, I have always found it to be a lovely resort for those with children, again with a pool with waterslide and a new water play area. You can also consider
The Villas at Disney's Grand Floridian Resort & Spa, which are on the grounds of this resort and give you the flexibility, as does Bay Lake Tower, of a deluxe studio or one-, two-, or three-bedroom villa.
Of the resorts that you mention, I would probably choose Bay Lake Tower at Disney's Contemporary Resort. I just love being able to walk to Magic Kingdom, and I really enjoy the theme there as well. I hope that these thoughts are helpful for you, Alison! Please do not hesitate to ask if you have any further questions.