How do I get my 3 and 4 year old to fall asleep at night?

I have a 3 year old son and a 4 year old daughter. They will not fall asleep when i put them to bed, they laugh, play, fight scream and argue with me about going to sleep. I've tried everything, sleepy music, staying with them till they fall asleep ignoring. homeopathic sleep aids, NOTHING works! Please help im at my wits end!