I think calling oneself “a child of God” in the way you describe does indeed require choosing Him and repenting — but that is in terms of being aligned with Him and being in relationship. To say we are all God’s children and all God’s creation is, for most people, one and the same. And that’s not wrong.
My point is not to paint God as an “all-forgiving flower child,” (and really, is it kind or helpful to be condescending towards another believer to make your point?) but that so many Christians do not see and treat “non-believers” with the compassion, dignity, and respect that Jesus commands us to do and have towards them. We don’t need to entice people over to the winning side, we need to convince them that they have a place reserved there for them, and that we and the Father Himself long for them to accept that invitation.