Given the choice between an expired high back booster (cosco something or other) or a not expired marathon where the straps *might* be a little below the shoulders, what do you think is the best option for an almost 5yo, 45lbs? In reality, i'm not certain the child is too tall, but he MIGHT be. If he's not too tall then marathon is hands down best choice. my dd is too tall but she is a few inches taller than her cousin. Last time I picked up my nephew and asked the preschool for a seat I was given an expired booster. I'm thinking this time it might just be better to see if that marathon fits him! His reaction will be interesting.. I bet he cant even remember the last time he was in a 5 pt harness :/ Hope he doesnt complain. I might pack treats in case bribery is necessary!
If the child has outgrown the Marathon, he outgrew the Cosco High Back booster a long time ago, probably several years ago (and has obviously outgrown it by weight. And in booster mode it creates a deadly fit, the expired issue completely aside.
None of those choices is safe, legal, or acceptable, and there is no way to say which is 'least bad'. All of them are bad.
If you want to continue transporting this child in your car, I'd recommend an inexpensive backless booster. Walmart carries the Harmony Lite Rider, and Target carries the Cosco Top Side. Both are less than 14 dollars (US seats). None of your other options cut the mustard, (unless of course he actually does fit in the Marathon.)
Yes, either every state or almost every state has a proper use clause in its child restraint law, so using a seat against manufacturer's instructions is against the law. This includes using a seat that is outgrown or expired or damaged in any way. If you read through the actual text of the law you'll see it in there somewhere.
I understand why you were asking, but there's just no way to know. BOTH are very dangerous. Since this keeps happening and he obviously won't fit in the Marathon forever, your best bet is to keep a cheapie backless booster in the car :-)