It would be a good idea to put this in writing to protect both of you against future accusations. He could say that you have given him custody and you could say that he hasn’t paid child support…Not that either of you would…but that option would be there and it would just be your word against his.
I will say that a month is not a very long amount of time and that it can sometimes take longer than that to get used to a situation. You may want to consider extending this time a bit. I understand wanting them to see that it may not be better with the other parent and that may take time. If your ex does not currently have rules or boundaries, that may change after a couple months of having the children rule the home so to speak. He may need time to figure out what he won’t allow also…Maybe you could begin this a month prior to the start of school and run through the first month of school? That’s going to be a transition phase anyway. Maybe during the summer they would be with you during the day and with him in the evenings, except those two days you mentioned.
You should also put into this what will happen at the end of that month. Are you all going to sit down and discuss problems or issue or is it going to be assumed that things are going to continue after this time frame if all is well…
If you and the ex get along well enough, you could also look at joint custody with equal time. The girls could switch week to week with you two…