It's always hard to know what to do in these situations. I usually bite the bullet though and say something in cases of blatant misuse or a situation that is inherently dangerous... sometimes the parent accepts help, sometimes they get offended.
Don't beat yourself up over it. It's never easy to decide.. maybe if you see them in the park again though you could build enough of a rapport to mention something...
I recently had a positive experience in a store where I mentioned to a mom a couple problems I noticed her infant seat had. (Dad was parked in front of the door, baby was strapped into the seat and I walked past them on my way in.) Mom was concerned and when she found out I was a tech asked me to come check her seats. Both her and dad were very grateful for the help... I always feel good when it goes this way. And when the opposite happens, I feel annoyed and frustrated that a parent wouldn't care enough to do better...
Anyways, I guess I'm just commiserating with you. I think it's hard for techs and advocates when we notice misuse, because we feel like we need to tell the parent and help them fix it. But in the end, if we spent time and worry about every single seat we saw with a problem, we'd worry ourselves to death.
There's a balance in there, and I think each person has to find it for themselves. You did what seemed best in the immediate situation, try not to beat yourself up now that time has passed and you're able to look back in retrospect. You know what they say, hindsight is 20/20.