I would cook lobster or mussels in front of my kids. We are very keen on not hiding things like where meat comes from, or death in general, from our children.
I find his swearing in front of his kids more offensive to be honest.
When our kids were growing up we regularly used to drive past an abattoir and although the elder girl knew exactly what it was for, the younger one, who was only about 5 at the time, was quite a sensitive thing and when she asked big sister, "What's that place?" closely followed by, "What's a nabbetwar?"' the answer she got, which satisfied her for many years was, "It's where animals go to have the meat taken out."
We were at an event recently where they were roasting a pig. Our oldest daughter (4) asked to see it close up, so I took her over. She asked what it was and I said it was a pig, she asked if it was dead and I said yes, she asked why and I said we were going to eat it. She was happy with that.
Water should be bubbling hot before inserting lobsters, crsbs or any live crustacions. It would have been more humane to have frozen them prior to cooking.