Upset parent with substitutions

U6 play 4x4 with 4, 5 min quarters. We have a team of 6. Every game 2 players will need to sit out 2 quarters. We are working that fairly. A parent was upset today when their child had to sit out twice. They are one of the “better“ players. It was stated that it was a waste of time to watch their child play only 10 mins.

How can this be handled better? We are first time coaches and had been doing this the last 2 games without issues. One thing is it was never mentioned prior to this since we never ran into push back from other parents.