I don't think they would have gone to Hogwarts, or at least Merope didn't. Merope seems so hopeless at magic, and Dumbledore gives the reason that she was terrorised by her father. Away from Hogwarts most of the year, Merope would be free from this and would be able to flourish and so even if she did struggle a little with her father there, I think she would be much more competent and confident had she recieved an education at Hogwarts. Marvolo also calls her a Squib. Getting into Hogwarts would be proof that she wasn't one. Dumbledore also wonders if Marvolo had ever learned to feed himself, suggesting that there was always someone there to do it for him, either his wife, or perhaps Merope herself. Maybe Merope wasn't magical enough to get a Hogwarts letter, but I'm inclined to think both Morfin and Merope were home-schooled.
I think Marvolo would have been arrogant enough to think that they were so pure in blood that they didn't need magical education. He doesn't seem to care for magical authority so perhaps he doesn't trust the establishment of the school. Perhaps he thinks that by accepting anyone, his children would be corrupted by going there.