T.G.
I think sentimental gifts should depend on the person receiving them (their likes, values etc)
Things Remembered is a store that engraves items such as picture frames, key chains, ornaments. You can find them online if you don't have a store near you.
Another idea is a Christian books store (since they are your sons Godparents.
Do they drink coffee? A very nice reusable to-go cup. You might be able to find a stainless steel one that can be engraved.