An affair is never the answer. Everyone ends up getting hurt.
The question is why he's changed. Many men feel left out after the children come along - they used to be the centre of someone's world and now they're just the wage earner because their partner doesn't seem to notice them any more. I know that's not how the wife thinks but men do see things very differently. I remember having this sort of conversation with a male friend almost 20 years ago. All of a sudden his wife changed from, in his mind, being an intelligent human being to someone who could only talk 'babies' and had turned from wife to mother. You need to be wife and mother - most men are just little boys at heart and can and do get jealous of the attention you give the children.
I've seen this with friends who have children - I honestly don't think the wife realises that her husband ends up feeling side-lined until things go wrong.
The best thing you can do is sit down and talk about how you feel - if you can't talk to your husband then who can you talk to? Remember it takes two to make a relationship.