I had a similar situation in that my party of 7, the 5 adult tickets would link but the 2 child tickets would not. I was assured it was no issue, just copy the FP+ selections to the childrens tickets when we arrive. Super, except that the kids would be riding TSMM during another time slot as our current choice was sold out. So I cancelled all the TSMM to choose a time when we could all ride together. None avaiable so now we all go standby (being able to check an attractions availability before canceling out entirely would havebeen very helpful here). I also had to give up our Illuminations and Wishes FP+ that week because the spaces were unavailable to add my niece and nephew.
FP+ and I were not buddies.