Well I don't have any kids so my opinion doesn't hold much weight, but I reckon that no sexual activities should not be practiced in front of the kids.

Once they turn 18 I think you should explain to them the way you are. But first ask them if them if they could cope with a revelation about yourselves. If they decide they want that form of lifestyle (and are scared to tell you) and come across you (for example) in a party, imagine their reaction.

In the poorer 3rd world countries, parents have sex in front of their kids, because there is no alternative.