Dear ABP, sorry to have offended you!
I work for an airline who generally have clear SOPs. It says the belts are for infants under 2, or fat people. As per Tiger's post above, there are some options for the under 3s, but no infant belts for over 2s. Yes, we've all had plenty of discussions with parents, and we've all been asked quietly by the parent to tell, nay threaten 'Rebecca' to put on her seatbelt (because they don't have the authority themselves).
The thing about SOPs is they need to be easy to understand, easy to remember, easy to communicate and easy to impliment. If I said to parent, sorry, Johnny is 4.1 kg and the limit is 4, parent would say he's just drunk a litre of milk and he's wearing his winter coat.
"Is he over 2 yet?" is much easier than getting out the scales.
If he's a 26 month old wriggler, it's up to the parent to ensure the seatbelt is tight and that, like in church, you're not supposed to be rutching and thrutching. If he's a big heavy 23 month old, he's still going to fit, because those belts are also used for fat people... and they're really long. But parents CAN, if they choose, buy a seat for him, and put him in an approved car-seat.
Yes, I acknowledge it's not easy for parents, and I also hear EWXY examples of when there are situations you need to go outside Standard Operating Procedures... when things aren't standard let's say... I agree, one temper tantrum 2 year old in an infant seatbelt is better than a go-around, but to echo what EWXY says, it needs a clear explanation to the parents that it is an unsatisfactory situation.