Church ruins

I want to say:

The boy was sleeping in the church ruins.

I came up with:

Puer in ruina ecclesiae dormiebat.

‘ruina’ is in the ablative, because of in (in + abl.)
‘ecclesiae’ is genitive, because it is the ruins of the church

I think that is it, but I’m not sure.

Should ruina be ruinis?

I’m not sure.

I found this definition for ruina:

ruina -ae f. [falling down , collapse, ruin, destruction; the ruins of a building, debris].

So I wasn’t sure if in ruina meant (in English) in the ruins.

The alternative phrase would be:

Puer in ruinis ecclesiae dormiebat.

ruina is generally used in the plural when it means “ruins” of a structure or something similar. Mind you, there are other ways to say “ruins” and words meaning “remnants” or “remains” are good (and sometimes more striking) alternatives.

So my alternative

Puer in ruinis ecclesiae dormiebat.

is the correct use of ruina?

yes. Cicero doesn’t seem very fond of the term, but since you are talking about a church, a church that has even had time to become ruins, that doesn’t seem to be much of an issue. but if a bishop were to see the boy sleeping in the church ruins, he would no doubt use the phrase “ruinas penes dormiebat.”