Several months back, there was a long thread where what you're talking about was discussed. As far as I can remember, the consensus was that the previous practice ("loophole") of only needing FP+ for the first group was being eliminated. (There was much debate over whether parents should be allowed to "double up" on FP+, or if it was justified bc those of us with small kids have overall less touring time bc it takes double+ the time for two groups to go through rides....)
We've been since digital rider swap was instituted, and at most of the rides we went on (including I know at SDD, 7DMT, BTMRR, and Splash) CMs made sure everyone in our party, including both the first and second sets of riders, had FP+ before they would make the digital rider swaps.
The practice was to check FP+ (everyone has to be present, including the child not riding), then the CM would replace the second groups FP+ with a new time (usually set to begin when they expected the first group to exit the ride).
You may have a different experience. Not trying to be judgmental, just wanted to warn you they they may check that everyone has FP+.