Of course it should be an option to want to place a kid in a Christian home.
No, it shouldn't. Adoption should be about finding a good home for a kid, above anything else. A family's religion should not a determining factor in whether or not they're suitable to adopt. There are plenty of non-Christian families with good homes who want to adopt and care for a kid — to make a kid wait longer to find a Christian home than they could've had to wait if non-Christians were allowed is a policy that should disqualify someone from running an adoption agency.
Some people believe that a Christian home is a good home. Maybe you disagree, but you can’t bully people into following your beliefs with the law.
Fine. But they shouldn't receive public funds.