Given the seating in CE is 2+2, you can't really all sit together anyway. Of course you could sit across the aisle from your folks and maybe that's worth £88 for peace of mind.

There is a risk that you can pay to seat them near to the seat you've selected, but then either you or they get seat shifted. Maybe it's not a huge risk, but it does happen.

The CE cabin is rarely very large, so even if you don't pay you wouldn't be very far away and if they're next to each other maybe that would be enough? Best case at OLCI you can pick three seats near each other anyway.