Me, I’m allowing this (both the U11 & U13 girls scenario)
At this age and level, its all about participation. Do this, and the youngsters will enjoy themselves more, and learn more. Don’t do this, and someone from the losing side may well not bother next week as they’ve had a miserable experience and could be lost to the game forever.
As a bit of an Aside, when I first started, I was predominantly doing youth (U15 & U16) games. I remember having a number of conversations with managers, club secretaries and the league, suggesting that goal diffference not be used to decide league position. One club, very well run (and I am pleased to say, went on to win the league) would always rotate their better players off once the scoreline was running away in their favour. Some of their rivals would try and rack up a cricket score when playing the weaker teams in the league as they wanted to improve their goal difference, leading to some worthless encounters that benefited no-one.