You want them to be born in September or October (or, more generally, a month or two after the school year starts). Probably October, so that there's no chance they're born in August. They'll be more developed than their July birthday brethren at every stage of their education - it's a very easy boost. For example, exams at 16 in the UK (GCSEs) that by and large everyone takes, you'd expect 54% of autumn birthday children to get 5 grades A*-C. You'd expect 48% of the summer birthday children to achieve this. At age 11, the gap is a slightly larger 8 points in terms of percentage of children who meet expectations.

This is a nontrivial difference, which makes it a trivial decision.

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