Bl**dy hell TW, this is a DISGRACE.
Oh no, but of course, I forgot, Its "THEIR" day which means sod the feelings of anyone else, ESPECIALLY our guests, particularly the ones who are close family.
Am I right in thinking that firstly, your two older children were asked to perform duties (which naturally any young child would be thrilled about). Once they were baited, only then were you told that your youngest child was not welcome.
This has put you in a dreadful position and I can't help thinking that they have been highly manipulative towards you and your children in the quest for "their perfect day". Sorry to be so derogatory about your relatives as I'm sure usually they are lovely, but their attitude here is shameful.
You have now been driven along by the tide of events, being polite, trying not to make a fuss, hoping that someone, anyone, would see sense and realise that this is definitely not on.
I agree with www, that the time has come to put your foot down.
I'd say something (or write a letter if it feels better) along these lines:
Tell them that you have been extremely uncomfortable about this from the beginning. You have tried very hard to go along with what they want, but this is "guest list rules" gone too far. You have always treated your children equally and you can't in all honesty start treating them unfairly now. You have respected their wishes about excluding 1 of your 3 children from having a "role" in the day, despite it going against your family values (ie fairness). But, its pushing those values too far to expect you to leave 1 member of your family behind for a weekend(who also happens to be a two and a half year old little boy)whilst the other 4 attend.
Anyone else at the wedding (whose children have also been banned)would COMPLETELY understand why the baby brother of the bridesmaid and page boy were there. Remind them of this, as I think this is where this half baked, batty and downright insulting idea comes from.
If you say all this and they still won't budge, then simply take him anyway. If anyone kicks up a fuss on the day, then be straight and tell them what was asked of you was unreasonable.
This may be the only time any of your children are asked to perform this task. It is indeed a special day for them, one which ALL of their close family should share.
This has taken me ages to write, so forgive me if I've repeated what others have said etc.
Good luck.