The difficulty I have found when officiating girls’ fixtures (although have only done U16 & U18 girls, not U13) is that you can lose some of the “player generated clues” when a ball goes out of play.
I.e., certainly in men’s OA, and in much of boys football, a player will only go and get the ball for a throw/goal kick/corner if it is their throw etc.
In the girls’ games I’ve done, often the player nearest the ball goes to get it. This can be surprisingly confusing (it shouldn’t be, and I applaud the girls for their sportsmanship) - just be aware of it: make your decision and stick to it - maybe keep your arm out indicating who’s throw that bit longer.
The game will probably be quieter, the spread of ability amongst players probably greater: you’ll possibly get a few very, very good players on a team, through to someone for whom this may be their first game of football. Enjoy it!