= Solution
Treating the two-year observation period as approximately complete, divide total <person-time at risk> by two to estimate the average number of children being observed. Dividing again by the number of families gives
$$
\frac{3595+3888}{2(1700+1700)}\simeq1.10.
$$
Thus the estimate is \b[about 1.1 eligible children per family]. This is a person-time-based approximation for children contributing to the study, not a count of older children or an exact reconstruction of family composition if follow-up or eligibility changed.
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