Toontown opens an hour after the regular park (and 2 hours after EMH/MM if it was an early entry day in DL) and that is my favorite time to get in quick meets. We love to head up to the ToonTown gates about 5 or so min. before it opens (use the Fantasyland Theater restroom on the way).
Then usually just a minute or so before opening some of the characters will come out, mingle (usually with the kids that are waiting) & grab one or two of them by the hand and lead them to their first meet location.
For Mickey & Minnie that will be their houses and if another character doesn’t walk our child to their meet then we’ll hit Minnie’s first (her line can get long very quickly in & doesn’t relent all day) and then Mickey’s who has ways of making his line move faster if & when the line builds that Minnie doesn’t and never seems to build as quickly. We then hit up Gadget’s & Roger Rabbit’s and I always try to make sure we have a FastPass (usually for Small World but sometimes Big Thunder or Matterhorn) to help us usher the kids out of ToonToown with the promise that in the evening when lines are longer we can return for all the playing/exploring in ToonTown that they also love.
I find that the lines in DCA - Buena Vista Street and also the lines in Town Square in Disneyland get really mob-sceney quickly for Mickey & Minnie (less so than the others) so we haven’t met Mickey or Minnie in either of those locations in a long while. People will use the app to stakeout what time to come hang out around the entrance where the characters will appear and then there’ll be a rush to create a line. If you are particularly fond of their different outfits in DCA (and sometimes in Town Square they will have a specific celebration outfit- like they do right now for Mickey & Minnie’s birthday) then it’s worth it to do the stakeout as well. For example, we loved the Diamond Celebration 60th Anniversary at Disneyland & so for the last morning of that event we made a point to visit Mickey & friends their last time in their Diamond outfits & it was a fun morning, but definitely some lines & stakeouts were involved & the only one we had to come back at a different time for was Daisy because she didn’t make her first 2 times posted on the app.