Realize ahead of time that because you have a four year old and a six year old that want to help, that you are going to have to do some strong mental preparation.
While they are are school and pre-school, assemble the tree itself. Make sure that all the lights are working and that the tree is in the position that it will be for decorating. When they get home, you will be ready to decorate. To them, decorating is actually putting the ornaments on the tree. It's your task to do all the ground work in preparation for actually ornament hanging. Get over it.
Lay out all the ornaments. Remove any ornaments that you absolutely do not want them to touch. Hang them on the top 1/3 of the tree where they will be out of reach. Option 2 would be to put them to the side and wait until the ornaments are going up and you put them on the top 2/3 of the tree. The top 2/3 is a critical space for parent's ornaments.
Relax. Take a deep breath and close your eyes.
As a mother, resist the urge to become a tearly ball of emotions as you unpack all the ornaments from the past 22 years. It's productive to remember the holidays and how your kids grew and the special memories of those holidays as long as you keep it in perspective and don't break apart. Do NOT focus on what didn't go right or what you lost. Focus on what you have and what you are thankful for. Think about those that are no longer with you with gratitude, not loneliness. Your bank account balance, your current job situation and your time of the month do not "control" your tree decorating festivities.....unless you let them. Don't let them. Did you hear me? Don't let them. These things are adult things and are not (or shouldn't be) part of your children's memories of decorating this tree. Keep those worries where they belong.....outside for just this brief period of time.
When kids come home from school, make sure they have a snack and finish their homework and go potty. Since they wash their hands after the bathroom visit, you are ready to proceed with putting the ornaments on the tree. You will have saved yourself a mental meltdown IF you went over some ground rules while you were having that snack. In our house, we call those rules "success rules." Success rules could include: only hanging one ornament at a time, being able to place at least one hand in between the ornament that you are hanging and the ornament right next to it, having to hang the ornament hook or loop of ribbon at least 2 fingers back from the tip of the branch (so the ornaments don't fall off and break) and making sure that each branch has at least 1 to 2 ornaments on each branch so the branch will not be lonely. And perhaps most of all - this is something that we all do together and it is NOT a contest to see who can do it the fastest. Resist the urge to allow your kids to argue about who can hang the ornaments the "best" or who has hung more than the other.
Realize that these are some memory making times and in the whole scheme of things, it matters not that the tree doesn't look like something out of a decorators magazine or that there are 4 ornaments on this branch but only one on the next one. What matters is that you made some memories with your children who still see this holiday (and all of it's preparations and glory) through the eyes of a four and a six year old and they will remember how you behaved. When they are grown and out of the house, you can have your decorator tree. My guess is that you will not want it then, you'll be pining for having all your kids young again and the opportunity to have them decorate the tree with you. Make the memories good ones now so that you'll have something to cherish later on.
When the kids go to bed, you can rearrange anything that is really bothering you...but there is something precious about a tree filled with 22 years of ornaments from each child's holiday and something even more precious about you decorating that tree all together and laughing and having a good time.
Relax, you're done. Next is how to survive putting up the exterior wreaths.
Get the bottle of wine :)