Your husband was not at fault. Unless you both did not make the rule clear enough-and all kids do this eventually when we least expect. Sounds like the kids knew the rules, and decided to do it even though your husband did not answer or "hear" the request, which they knew was not OK anyway.
Dads, and PARENTS are often oblivious and busy with a thousand things. That's WHY rules need to be followed. How many times have I been doing a million things that would make me look negligent if the kids got hurt or ran into the busy street we live on? That's why we have VERY strictly taught RULES as WELL as providing locks and watching all the time. Sometimes you forget to lock a lock. Sometimes you aren't watching kid A when kid B has just fallen off the swing set etc. Your kids should KNOW that the RULE to never leave the yard is a MAJOR one punishable by maximum penalty.
If this was the first time they ever did it, and they were really that sorry, then it may be OK to let them know that if they EVER do that again, then X (something SUPER SERIOUS) will happen. You know your boys. If you think that in the end, they might calm down and have fond memories of the cop car ride etc, it wouldn't hurt to assign a serious consequence now too, to super seal the deal and make sure they don't do it again.
For their ages I might not let them "play" outside for a week, but instead do some TOUGH chores every day. Or something very dear they lose for a week PLUS some big unsavory task they need to do. Not angrily administered, just as penance for a VERY seriously wrong action that risked their safety and scared everyone to death.
If you feel it would help make sure to deter future episodes, I would probably do that, personally.
I'm glad they were OK, how scary!
***Just read Riley's post-yup, I agree! And Dawn's too.