Because of dh's job, we all have first class travel, and we've taken all three boys on the train, in first class where available, all their lives - and we have never had anyone object to their presence, or glare, or tut at us. We have always made the boys behave reasonably, wherever on the train we happened to be sitting, and though they haven't been perfect, they have been acceptable travelling companions.
We took them in first class when they were babies, too - and that was never an issue. In fact, the only train journey I remember being really bad was the steam train from Fort William up to Mallaig - the train was busy, so one ds and I were in one pair of seats, and dh was further down the carriage with the other dses, but for some reason ds2, who was with dh, was playing him up terribly - so dh stormed up the carriage with him, dumped him with me, and took ds3 back down with him. Whereupon ds2 behaved angelically, and ds3 started to play up, so dh stormed back up the carriage to swap them again - but that trip was horrendous because dh wouldn't listen to any suggestions on how to get the dses to settle, but just kept swapping his badly behaving ds for my well behaved one (and yes, I was very slightly smug that both ds2 and ds3 behaved impeccably for me).
I don't think it should make a difference whether you are in first or standard class - you should still do your best to make sure your children don't make the trip hell for your fellow passengers - and I'm sure that's what CommanderDrool is going to do.
I have to say that I don't think travelling first class means you are paying for a quieter journey - but the fact that there will be fewer passengers and more space will mean it will be more comfortable and probably more quiet. Personally I like first class because of the bigger seats, to fit my humungous ample derriere in comfortably!