It's a little bush league at 10U to be sending runners to 2nd IMO. Just steal on the next pitch anyway and try to get a throw. I've never seen a 10U team defend it and not sure I've seen a 12U team defend it too well either.
But I would say the ump got it wrong as I understand the rule.
I guess I just don't like teaching girls plays/strategies that they will rarely if ever be doing when they get older or play at a higher level. My HS team will get an easy out if you try it and most of the teams we play will do the same to us. So why teach something at 10/12 that you won't be doing in 2-4 more years?
The most enjoyable time I ever had coaching 10U was when they had a no steal home policy. Made the league so much better. Had to actually earn your runs instead of waiting for the inevitable past ball.
Softball needs to amend rules a little better for the different age groups IMO.
I saw a bunch of imaginative baserunning in 10/12U "competitive ball" that gets you put out quickly at 16/18U. Most of it goes away, but people still try that running to 2B with a runner on 3B when the game isn't on the line, and more often than not, DD's team gets an out either at 2B or Home.