I think they are just using the term 'children' in a more casual sense.
It illustrates the problems raised by their pedantic theology. If the 'other sheep' are not sons of god why do they call him their Heavenly Father in prayer?
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We cannot pick and choose which of the above descriptions of those who belong to God apply to us, and which ones do not. We cannot for example be reconciled to God, but not be a “new creation”. (see 2 Cor.5:16-19) We cannot be “beloved children” of God, but not be “born of God”. We cannot draw a line between calling God “Father”, and calling him, “Abba, Father”. If we have not been adopted then we are not His children, He is not our Father, and we have no right to call Him such at all. If we are His children then we are also joint heirs with Christ. If we are “in harmony with the Spirit” then “God’s Spirit truly dwells” in us, but if we do not have the Holy Spirit then we do “not belong to” Christ. (See Rom.8:9-17) Only if we are “led by the Spirit” can we produce the “fruitage of the Spirit”. But again, if we are, then we can cry out “Abba Father” and we are adopted as sons of God and joint heirs with Christ. (See Gal.4:6,7;5:16-24)