Hi Paul!
I personally love the Dining Plan because to me, experiencing all the new foods and restaurants is as fun as a ride or attraction! And the Dining Plan lets me not think about the money and focus on the fun! There are several different kinds of Dining Plans and you can
see them here and even download full brochures. I've done the Deluxe Dining Plan with my family of 4 (my daughter was 4 like yours!) and we were very happy with it.
Which days to go to which park is a hard question---there are definitely days which are historically less crowded, etc. But there are also things like Extra Magic Hours and special attractions/events (like Fantastmic at Hollywood Studios) that are a draw to a specific park even if it is historically a crowded day.
There are even websites (like touringplans.com) that predict which parks will be the most crowded each day. Or you can be like me and base your day at the park on which dining reservation you get! (See, I told you I loved food!)
Have a magical vacation! And happy birthday to your 4 year old! :)