In Aug, I adopted 2 children from China who became citizens upon entry into the US. For answering time lived with me question, do I say all year like a birth or 5 months?

An adopted child is always treated as your child, according to tax law.  This from IRS Publication 501, Exemptions, Standard Deduction, and Filing Information:

Adopted child. An adopted child is always treated as your own child. The term “adopted child” includes a child who was lawfully placed with you for legal adoption.

You may say they lived with you all year, regardless of when they arrived.

Does it matter if the child was born in 2016 and prior but adopted in 2017?

No, just like it wouldn't matter if your birth child was born in July 2017.  She's considered having lived with you for the entire year.