From the ages of about 2 to 7, the characters were more important to my DD than anything else in the parks. (At age 10, she still loves them- but since she has pretty much met them all multiple times, she does not absolutely have to meet each and every one at every location every trip now). I recall one MK day where we went from character to character almost the entire day and maybe rode 2 rides. She still calls that the best day ever (DH and I would differ). Also-- at that age, even if my DD had just been to CRT and had seen Cinderella- if we passed her meet and greet in the park (or really just after the first time when DD knew she also met in another location at MK) DD wanted to see her again. If they were the favorites- she would want to see them again at any location. Also, if they dressed different, then she would want to see them at all such locations. For example, at MK, Minnie is now in pink, Epcot- traditional, AK- Safari. All were necessary.
As the PPs have said, Kenny the Pirate is wonderful. His app and his recommendations have saved us a ton of time. You want to do some research into which characters are the most popular and plan accordingly. For example, there are a ton of characters around WS in EPCOT--- which is one of the reasons that EPCOT was DD's favorite park other than MK when she was a preschooler (still is today). For those characters, you want to know their times and plan a path where you get to the area and get in line 10 to 15 minutes before the character comes out--- especially for Belle, Jasmine/Aladdin, and Mulan. (And don't think that Akershus will get you out of Belle, because she wears her Gold dress in Akershus and her blue dress outside). Those 3 lines particularly, often get cut off right after the character steps out (and occasionally even before). The minute the character comes out and people see them in WS the line builds extremely fast. Really for most characters that have intermittent meet and greets in any park, your best bet is to get there at least 5 minutes and preferably 10 before they come out- that avoids the immediate long line build up. For Joy and Sadness in EPCOT, you want to head there immediately at Rope Drop because that line builds to 45 minutes to an hour within the first 30 minutes of park time and stays there (and don't be swayed by that cute Baymax if he is still there, his line does not build up as fast- see Joy and Sadness first). I don't know if you have a night owl kid (I do), but if you do extra evening hours at Epcot and wait about 20 minutes in to let the Joy and Sadness line dissipate, the line is much shorter than during the day after it builds up.
MDE also now has character times and locations and it can be useful during your day, but we found that Kenny was often more accurate.
For us, we had to do game plans to meet all the characters, and that was the most important part of DD's day those years. I would say at the very least, know who your DD's absolute favorite can't miss characters are, and do some research and come up with a plan for those. If you want to post who the most important are, I may have some ideas for you.
You mentioned Ariel's Grotto and the Character Connection. Ariel's Grotto- you can find low line times throughout the day if you keep an eye on it- I don't think we have ever waited more than 10 minutes for her. If you go in the first hour, you probably won't wait more than 5. We typically go straight to Peter Pan (have FP for 7DMT some point in the day -or we would go to 7DMT first then Pan if the line wasn't bad yet), then Pooh, then go see Ariel- and we often walk in. We have also waited to see Ariel until night time and walked right in. For the character Connection at Epcot, it isn't as bad as it used to be. It used to be an hour most times when we were there and now we often find it at 20 to 30 minutes which isn't bad for multiple characters. If you are heading straight to Joy and Sadness, then hit Baymax, then go to the Character Connection and the line may still be 15 minutes or under by the time you get there. If Joy and Sadness aren't of interest, if you go to the character connection in the first half hour at EPCOT, the wait typically isn't bad. Of course, all that depends on whether you have a FP for the Frozen ride, how many days at EPCOT, if you used your TIER1 on Frozen and are trying to rush to Soarin or Test Track at rope drop etc. It is all about priorities with those annoying Tiers and that annoying FP+!
I will say though it involved a lot of lines, a lot of waiting, and a lot of time- those pre-school character interactions are probably our most precious times at WDW and our best memories.