Cinders advice might be different where you are but current advice here is to only treat the heads of those individuals who have proven lice or nits (the eggs). This is because the lice can (and do) become resistant to the treatment and so eventually it doesn't work anyway. It is really important that you check everyone's heads regularly though (including getting someone to check yours)
I currently manage a head lice scheme in 90 community pharmacies (if they can prove there are definately lice by bringing at least one in per individual who needs treating on a piece of sellotape) whereby we supply the treatment free. Again I don't know if you have such schemes where you are but it might be worth investigating for future b/c from personal experience if your kids have had them once while ever they are associating with the same children they will definately have them again
Me 43 XH 45 M 2.7.88 Divorce 7.10.09 Kids D20,S17 & D15