Hi E., My son turned 5 on July 31 of the year he was supposed to start K. Pretty much everyone except his preschool teacher said to hold him back, including my aunt who was a long-time teacher. They said esp. with boys, not as much with girls, they are too immature. He still took naps and to this day (he's almost 7) still gets sleepy sometimes in the afternoon...he is my big sleeper, but it hasn't been a problem.
We also debated because we have another boy who is 23 months younger so that would have made them closer in school (now 5 and a June birthday.) The oldest boy's PreK teacher said he could have handled K in December of the 4 year old year so I sent him. I didn't want him to be bored and cause behavior problems, even though he is not super-wild or anything, he can be silly if bored. I decided if he has trouble, he can repeat K without much difficulty.
Well, sorry this is a long story but I put lots of thought into and agonized...Results: He did GREAT! His school does "Flex" reading and math and he was ahead in both. I remember getting newsletters from the homeroom teacher saying they were now working on "letter G" or something and I thought oh goodness, my poor son would have been miserable even in that K reading class. He is not developmentally behind at all, now almost 7. All of his friends have birthdays before him and he has to wait until late summer, but otherwise you would never know he is still 6. He is not small or uncoordinated, keeps up and even exceeds many students...and he is just a regular boy (no super-brain or anything.) He plays sports and really likes school.
Now he was tired after school, but I would not say we had any more behavior issues than normal. Just give her an earlier bedtime if she is tired. I don't even remember seeing much of a change in his sleep or behavior at all really. My son still goes to bed at 8 as normal. He really adjusted automatically right along with everyone else. He can even handle after school activities.
We also have a 5 year old now coming up for K this fall and we are sending him (with the June birthday.) Same reason. Some students who have not been to preschool at all are starting from the beginning. But also, if you prefer she is the older one in the group and want her home another year, I don't think you will harm her to hold her back. I guess she could skip a grade if necessary. I just don't think it is necessary unless you prefer it, she isn't ready, she is very small or uncoordinated. Eventually they all catch up to the same basic level. It's just that the first couple of years you have many different levels. People still tell me about holding children back and after this experience I just can't agree. I can't imagine what he would have been like for mine to have started a year later. There were many students with late spring/summer birthdays in my son's class. It was definitely the right decision for us to send him, but it is a family decision. Make sure your husband agrees. She'll adapt either way. Don't let people scare you. Is she emotionally ready for school? Do you think she'll have separation issues? I cried when my son went, but he did not. He was excited and has never looked back. They have to be ready as a whole person.
We also have a girl who we planned to start school 3 years after our second son, who will now get held back against our wishes because they changed the deadline and she is a September birthday so I guess I'll get to experience that side one day. I know how you feel too, as I am happy to have my last child home another year and am not in a hurry to rush them all off...school does make them grow up fast, but given the choice, I'd rather have her go on in 3 years behind her brother if she is ready.
Hope that is helpful to you. That is my story...but your daughter being in full-day preschool, she is probably more than ready to go! I think you'll be glad you sent her once she is there and doing well. My boys both went to half-day preschool. Feel free to email me if you have questions. Sorry to ramble on. Best Wishes, M.