Hi Danny

So I took a quick look at family search and I am afraid that the parish where Seferino Alonso's grandchildren were baptized has records only as back as 1799.

I would focus on finding the baptismal records for all of Seferino's children, if you believe they were born in that same town of Salinas.

If you truly want to find them you can go page by page in around the years you think they were born. I did this and I was able to find my ancestors who had been indexed incorrectly in family search.

Good luck