Agreed with everyone on keeping it fun! Make sure there is a lot of variety to the lessons. Make little details an integral part of your teaching - yes they are young and high strung at this age, but they are also little sponges, and setting up their future outlook on handling horses starts NOW, so keep in mind the major impact you are having on these children as future horsemen. Teach them to pick up on and read their horse's body language by playing "guess what my horse is thinking" games. Sacrifice 20 bucks out of your own pocket at the local dollar store to give them prizes, stickers, incentives for doing well. Since they are gung-ho on competition, utilize that as a teaching tool - you have 4 kids, all the same age and relative skill level - use that to your advantage! Set up easy, mock horse shows where everyone wins something.
Those that are more timid, don't push them too hard. Be willing to do some lessons from the ground and work your way up!
The biggest thing that will help you, I think, is changing your own mindset. I was much like you before I had my own child - like, oh god, kids. But since a lot of times the 6-10 year old crowd was my bread and butter in lessons, I thought of it instead of teaching brainless children, think of it as teaching the future horse generation - what are the things that you cannot stand your current horse peers not knowing? Remedy that for the future!
Also set some very hard and fast ground rules that are total and complete - you scream, you're off the horse - grounded for the remainder of the lesson. You yank on a mouth - you're grounded - you fail to listen - you're grounded.
Good luck, and believe it or not, you may soon find you are loving it in spite of yourself.