The content of what you're saying isn't necessarily bragging, it would be down to how you say it.
So an old friend with a kid or two walks up and says, "So what have you been up to lately?"
Well, thanks to not having children I have really nice holidays each year and a really nice house! What have you been up to???
That's bragging, the way it's said, even when reading it, sounds like gloating, "I have something you don't". Just changing a few bits changes it from bragging to just chatting.
Well, I've got my own place, I work and go an a holiday or two a year, but then again, I haven't got kids so it's easier to do.
Both have the same basic content in what is being conversed, but one sounds arrogant and the other sounds sympathetic to family life. If you decieded that you don't want kids because it would effect you current life too much, be honest. If you add on "I just don't think family life is for me", it still sounds sympathetic and it shows you've thought about it at least briefly and decided to take a different path.
Body language as always plays an important role as well though. Provided you don't think you genuinely have the better life and therefore you are not trying to brag, by not bragging, your body language will portray that message as well.
A bragger generally makes themselves taller and looks over confidently at the person they are speaking to, while grinning derisively at them, while looking like they don't really care what the other person does or says. A caring person in conversation looks relaxed and interested in what others have to say.
Something also worth noting is taking the time to listen to them. If you explain about your life and then don't listen to them, that's definitely going to come across as arrogant and as you bragging, believe me. If they are parents, their personal time is far more important to them than yours is to you!