I think the food is similar at both places. It really depends what characters you want to see and if you want to leave the park to eat. With your DS being just 4, I would pick Crystal Palace. Pooh and friends characters and not having to hassle with leaving the park!
But you will like both equally I think.
We did Crystal Palace in December 2017 and just did 1900 Park Fare on Sunday, Dec. 1. For us, the "pro" of an early breakfast at 1900 Park Fare is that you can Uber straight from your hotel to the Grand Floridian and have breakfast, then monorail to the park. Agree with earlier posters that the food is similar. We were a party of 8 on both occasions and liked the round table we got at 1900 PF as opposed to the rectangular table at CP.
I really like Crystal Palace! The characters from Winnie the Pooh are always a hit across all ages in my family, and the restaurant building is spectacular. If you’re not staying at the GF resort and your kids aren’t particularly obsessed with Cinderella, I don’t really see a point in going out of the way to 1900 Park Fare.
Crystal Palace - We have been about 3 or 4 times, and always tried for 8am ADR when MK opened at 9am, as you can't beat an almost empty Main Street, we got great photos in front of the castle without a single person in the background. We always enjoyed the food, especially pooh's puff toast and breakfast lasagne. The characters are enjoyable too, especially Tigger.
1900 Park Fare - we have only been once, but would definitely return. The food was excellent, although I can't think of any 'speciality' items they had. The characters are enjoyable, especially the Mad Hatter, he was very funny and the interaction was more than just a photo opportunity. Alice didn't come to the table, you had to go to her so we didn't bother. Mary Poppins was divine though
We have enjoyed both of them. I would think with a 4 year old, Crystal Palace might be more their speed and if you get an early adr, then you can get those in front of the castle pictures with hardly anyone in the park.