I think possibly there were conversations about where Helen was on the Monday. And IS gave a different story because he hadn't thought about the story of the note yet, like 'she's out walking the dog' or 'I don't know where she's gone', or 'she's gone to bed because she's been having sleep problems recently'. I think they might be protecting their dad by saying there was no conversation about Helen on Monday, and that he implied to them (on Tuesday) that Helen had left (and left a note) on the Tuesday. I think IS had a story of Helen vanishing while out with Boris, but then changed that to Broadstairs because he suddenly got very nervous that police would concentrate on the house. It's perhaps one thing planning on using the perfect place to hide a body and quite another, after you've done it, and start to get paranoid about being caught.
Another piece of evidence that supports his change of story is him telling the solicitor that Helen was unwell. He could very easily have said she's gone away for a short break.