Answer
(Fatwa: 1093/919/B=1432)
The Imam is not right in saying that salah shall not be valid until minor child does not go out of the mosque. The ruling regarding the minor children is that their saf (row) shall be made behind adults. But if there is space in the saf of adults and any minor child comes he can stand in the available space.
إن لم يكن جمع من الصبيان يقوم الصبى بين الرجال (مراقى الفلاح 308)
Likewise, if there is only one minor child he shall stand in the saf of adults.
يصفهم الإمام بأن أمرهم بذلك الرجال ثم الصبيان ، … فلو واحدا دخل فى الصف (رد المختار 1/568)
Allah (Subhana Wa Ta’ala) knows Best
Darul Ifta,
Darul Uloom Deoband