That's a good question that won't come up very often. (I assume an overseas adoption)

If you are using miles, just book one-ways, as that would be easier for all.

If paying cash, you might find that one ways are much more expensive. But if not, again, just book one-ways.

But because infant tickets are separate, but attached to, an adult ticket, I would have thought that you could just book the infant ticket on a one-way basis. However, with TK you are going to find a variable level of English skills and competence when dealing over the telephone. If you could visit a ticket office at your nearest TK-served airport, that would be much better...