You don't even need certificate or proof of adoption. All you need is a court declaration stating you and your spouse are the primary care givers of the child and have assumed all financial and medical responsibility for the child. Depending on your country, you'd have to get a letter of no objection from the biological, but as far as EK is concerned, the child does not have to be adopted by you, but you have to be the primary caretaker, attested by a court, for them to be issued a family visa