Yes, “they were” is better. I can imagine “It was the great white ox carts …” being correct in some contexts if the sentence before was talking about “a sight” or some other singular noun.
“They could not believe the sight that appeared over the horizon. It was the great white ox carts …”
Something sounds odd in your sentences because they switch from the singular to the plural - he promises one cart but I guess he delivers multiple carts?
For me ‘it was’ doesn’t make sense in that context. The father refers to one particular cart and then there is reference to more than one. I can only assume that the writer is talking about a certain type of cart of which there are many examples. The only way to address this anomaly is therefore to write -