This is something I feel very strong about ... I am a mother of 3 children, and I don't like taking my children to weddings. My kids are 12yrs, 11yrs and 5yrs and generally well behaved (especially the older 2). But weddings are just long BORING days for them.
Think of it from a child's perspective ...
-you have to sit thru the long boring church ceremony, can't talk, can't move
-then there is the photo shoot between the ceremony, the parents really should spend this time letting the kids rest before the big night, but most kids want to run around because they have been expected to be quiet and sit still thru the ceremony for so long
-then there is the start of the reception, which means standing around int he foyer (again boring for kids)
-sitting at tables and waiting for your food, which is generally served a LOT later than most normal families would eat (meaning hungry and restless kids)
-sitting thru speeches (again sitting still, no talking)
-until FINALLY they get to get up and dance, however by this stage it is generally past most kids bedtimes
However I really think this needs to be a blanket rule. Right now I am disgusted in my FSIL (FH's sister) who has NOT invited my children (they are not FH's children) to her upcoming wedding, however she has invited cousin's children (who are both younger and older than my kids) and friends' kids. It seems that most children are invited except mine. FH is in the bridal party, but I am very tempted to say I am not attending (however with all the issues within the family, this will look extremely bad on me).
I absolutely agree with all of this. Its such a long day for a child of any age that I think the kids would actually prefer to be left at home with a babysitter.
In the case of your FSIL though, you can't invite one person's kid and not another! There needs to be a blanket rule across all persons invited, either you invite all kids or you don't.
We didn't have kids at ours. Fortunately not alot of friends or family had children so it wasn't an issue but I probably still wouldn't have kids if I were to get married again today.