is it just the kids bit though? when i lived with her we had a young baby and we was still very active after she was born, i remember a period of time though long before she was born where we must had done it 3 times in 4 months.
I think that the sex drive diminishes as you get older. Working full time, looking after a house and the kids, the kids getting older and becoming more aware, not having a lot of quality time together, illness, tiredness, stress and pressures of life all get in the way of a good sex life.
You have to find the time to make your wife feel special, relaxed and not worrying about what to make for tea or the kids' homework or the shopping or a million other things she has to do. You can't just walk into the bedroom and say "Fancy a shag love?". Well, you can but I doubt you will get anywhere