You can make an ADR anytime. You neither need to be on the dining plan or staying on site to book meal reservations.
We did just that an took a 5yo girl to dinner with Cinderella during an off site one day visit. We did the park one day and went back to the Grand Floridian for dinner the next. The meal characters have changed since we went but we loved it.
My 6yo nephew loved the Chef Mickey's breakfast and truth be told so did auntie and grandma. Mickey, Minnie, Goofy, Pluto and Donald were there. This year we are trying Crystal Palace and the Playhouse Disney meal at Hollywood & Vine, both for lunch.
