This was us about 9 years ago. We did this with DS #2 (out of 4 kids, also). It seemed like a good idea at the time because we had two in college and thought there'd be no problem paying it off as soon as DS #1 graduated. Then DH lost his job of 30 years and, luckily found another, but not nearly the salary he had. I had just gone back into the job force, working part time. So, we used the HEL to help pay and, as soon as we could, we rolled our HEL into our mortgage when the interest rates dropped. My job became full time and our income got back on track. We have managed to put the last two through with 529s, CDs, and a little help from Gpa.
I know DH really wanted to do this for our kids but I was very upset that we used the HELOC for college, though it sounded like a good idea when he had a great paying job and we didn't think we'd have any problems paying.
I think it would have been better to have let the kids take loans out, and give them money to pay it off. That way, they would have more "teeth in the game" and we weren't financially obligated.