Playgrounds are outside right?
Yep, but (what mccruiskeen says really), the smoking ban has made those who don't like smoke/smoking even more conscious of the smell of it. Even smokers realise how cigarette smoke smells now that pubs/restaurants/bookies haven't got a permanent haze of it inside.
Exposing your kids to cigarette smoke isn't a great idea, though given how many of us grew up being exposed to it and are "fine", maybe the dangers are overplayed, and anyway, as we all know, statistics don't matter when it comes to the safety and well-being of our kids. I'd hazard that exposing them to the sight of you smoking would seem to normalise it and even though they're "not allowed" to smoke, they may feel that it's something that they'll be able to do when they're older. I know it normalised it for me to see my dad and all his mates puffing away. Without exception, every single one of my peers had a go at smoking - some of us toughed it out and got hooked. Some of us saw sense and never took it up.
If you smoke outside in a playground, let's face it someone is going to get a whiff of your exhaled smoke. Chances are, despite that one whiff not really amounting to any harm, he or she is not going to enjoy it. So why would you want to do it. Just leave your child inside, go and stand downwind of the playground and have your fag if you're that desperate. Smoking in a playground doesn't make you an evil person, but it shows that you don't particularly care too much for the feelings of others.