How should I have handled this

[cardinalfan]

As far as the OP, some of this is just the product of u-10 baseball. There are a lot of inexperienced umpires who don't know the rules.
If you were calm as you said, there is no excuse in the way these two responded. One would hope they go check the rulebook, but more likely they are just there to pick up a part-time summer paycheck.

I wouldn't tell these youngsters that you have umpiring experience. I've been on both sides, and that won't help.
Instead, just study the book and when a play like this comes up, call time and calmly quote the rule. They still may be too immature or stubborn to change their mind, but at least they will know the correct rule for the next time.