Hello Julia and welcome to the Disney Parks Moms Panel!
As a grandma myself, I must say Bravo! to you bringing your mother-in-law on your Disney adventure. Multi-generational trips are a blast. The Walt Disney World Resort has many options for families of five. Many of those options can be found
here. For a family with very young children I always suggest you consider a monorail resort: Disney's Grand Floridian Resort & Spa; Disney's Polynesian Village Resort, or Disney's Contemporary Resort. This trio of resorts allow easy access to the beloved Magic Kingdom park.
If you prefer a less expensive option I would also highly recommend the family suites at
Disney's Art of Animation Resort. This beautiful resort is a toddlers dream as the buildings with family suites highlight three favorite films- Finding Nemo, The Lion King, and Cars. You will also find the largest swimming pool on the Walt Disney World Resort at this resort. Another advantage of the family suites... two bathrooms!
No matter which resort you ultimately choose, I know your family is going to have a magical time!
See ya real soon,
Anne