I would not bother with the finish line, gets rammed and very jossly and you can't see anything! I would go somewhere further out where you can see hime, less people, after Ditchlings Beacon somewhere? He can then proper wave at you and your children will see him in all his full glory after that bastard big hill!
Have you been to the finish line in Brighton and would you take a only just 4yr old, 6yr old and 9 yr old to it on your own. My brother is doing the race and I want to see him but also not sure about huge crowds.