I've said it before but I'm very much against holding back kids who don't have any real reason to be held back. Your DD doesn't seem to have any special needs. She's tall and mature. She's strong academically. You, yourself said she's ready. It's half day.... believe me, there is still plenty of time to have adventures with you and her sister before and after. My DD did half day and it was a nice to have some 1-on-1 time with my youngest. If at the end of next summer she really seems incapable of a full-day class, give her another year of kindergarten at a different school. I seriously doubt you will feel that way though.
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My eldest started at 5 and was so advanced and mature for grade they had to move her up to first after winter break for her own sanity. I can't even imagine how bad things would have been had we held her back. She's in 10th grade now at 14. No regrets. My youngest started full-day kindie at 4 and now he's 10 and a 6th grader. He's still considered the mature kid in class and still needs accomodation for advanced academics. No regrets.
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I don't know that school will be bad during a difficult family situation. It would be a stability that is HERS and seperate from any instability at home. Lots of kids really cling to the routine of school when things are stressful at home. Having outside adults you can trust and count on, can really help when you perhaps questioning a parent.
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It's sometimes hard to picture your babies in brand new situations. We often don't see how capable they are until they move outside what we personally expect of them.