I think I've figured out how to bypass the "grayed out" situation on FP+, to at least make Cinderella or Rapunzel fp+ reservations. I was unable to get past the grayed out situation when attempting to book my entire family (2 adults, 2 kids) all at once. However, if I went in an manually changed FP+ individually for each family member, I was able to get each member a slot in either the Cinderella or Rapunzel meet & greet (just in case either gets switched to or combined with E&A). The only glitch was no 2 members got the "exact" same time, but we're close enough. For example, my 7 yr old daughter is at 9:05-10:05, I'm at 9:10-10:10, but we could both arrive at 9:10 with that.
The trick I'm trying (not sure if it will work) is I signed up my 8 year old son, who has zero interest in princesses, for the 9:05 with Rapunzel (whereas his 7 yr old sister who IS interested in Princesses is signed up for Cinderella at 9:05). I'm signed up for cinderella at 9:10, my husband for rapunzel at the hour beginning 9:20. That way, if we get there and A&E are at one or the other, my thought is the kids could just swap bands, so that it works out that my daughter is in the A&E slot, and I or my husband will be with her.