In our family we serve the kids first - yes food is wasted because some mums over pander to their kids needs and assume that their kid will be able to eat everything she puts on his place but that just isn't the case. I too hate it when food is wasted, but what can you say to someone who insist on filling her kids plate when you know yourself that they won't eat it. I guess the best thing would be to just say 'eat this piece first and then when you want more you can have another piece'? Would that be rude?
I don't think it's rude to say that. In fact I DO say that to kids when serving up, don't care what their parents think about it. Seen too many kids served up too much food by their parents, only for it all to be thrown away.
If the food is finished, u can follow it up and say "good boy / good girl!" you finished it. If they want more then give them more.
CB, good for you for packing those kids food, that should send a message to the mum.