Ginny,
I recommend making reservations for any meals where you want a table service meal. Is it required? No, technically not. But can you imagine not being able to get into a table service restaurant for a meal, especially one that the kids might be looking forward to, simply because it was booked up?
Now with a pair of 4 year old, you certainly don't need 3 table service meals a day probably. For my 4 and 2 year olds I plan one table service meal a day, occasionally two.
As for a suggestion in the Magic Kingdom, I would send you to
Crystal Palace. It has a great buffet with a good variety of dining choices. The quality is greatly improved and really quite good. The entertainment (Winnie the Pooh and friends) is great, and the location (dead center in the park) is fun too!
I am glad to offer other dining suggestions (those are my favorite questions!). Just let me know when (Breakfast/Lunch/Dinner), where (Magic Kingdom/Epcot/etc.), entertainment interests, and any cuisine likes/dislikes.
I look forward to your follow-up questions!
Have a magical (and delicious) vacation!