Romans 8:16 — The Spirit itself beareth witness with our spirit, that we are the children of God.
itself = Himself — the Holy Spirit is a person
with our spirit — not "to our spirit." A continuation of verse 15, "we cry Abba, Father." This is our spirit, but it is produced in us by the Holy Spirit.
children — Greek word teknon from tikto, to beget, "born ones" — like the Scottish word "bairn."