I think people (apart from Freckle) are missing the point of this. The OP isn't saying that her children should be stuck to her hip and attend every single function regardless of suitablility.
She has said that her and her dh enjoy child free time just as nuch as the next person.
The point of this is that it is a small family celebration at the beefeater not the pub not a nightclub, not a meal in a swanky place which starts at 8pm. But at the Beefeater at 6pm which all the close family are invited to.
I would be quite upset if my children were excluded from this event, and let's be clear, I'm not an over reacting, children stuck to me type of mum at all.
The point of this is not to debate whether children should go to everything, and I like the OP do not take the dc's to every function or party, but this particular one is extremely child friendly and quite frankly it sounds as if SIL has made all the arrangement to suit the children and then not invited them!!
It would be totally different if the gathering was at night in a really nice restaurant, then I'm sure the OP wouldn't have a problem with the no kids thing.