From tikto (to give birth), teknon emphasizes the birth-relationship — one who has been born of a parent. In the NT it is used for both biological children and spiritual offspring. John 1:12 uses it to describe believers as 'children of God' — those born of God through faith in Christ.
The distinction between teknon (child by birth) and huios (son by status/inheritance) matters. John prefers teknon for believers — emphasizing the new birth, the family likeness, the intimacy of the parent-child bond. Paul sometimes prefers huios to emphasize the legal standing and inheritance rights. Together they describe the Christian's full identity: born into God's family (teknon) with full inheritance rights (huios). The cry 'Abba, Father!' flows from both realities.