you're not 30 seconds away. you are a stay in your seat and fasten seatbelts sign away from your children. so, no in the event of your child being ill you would not necessarily be able to get to them.
i've had the experience before where we've had to sit for an extra hour and a half on the tarmac without food or water at the end of a flight because there was a problem with the airport security being able to accept us into the airport. i've had unexpected detours mid air to another airport because of weather issues. i've been sat next to really unsavoury people. etc etc. i'd not leave my child to negotiate all of these possibilities without me because i fancied sitting in bc without them.
then there's the things that i've never experienced like someone kicking off violently and having to be restrained, hijacking, emergency landing, plane crash etc. these things all happen to someone even though they never think it will happen to them.
for me personally no matter how i tried to justify it to myself and say these things will never happen i wouldn't take the risk - you would never be able to live with yourself if an emergency did happen and you couldn't be there. those who are using the physical space as an indicator of how long it would take to get to them are being disingenuous - if the fasten seatbelt sign is on (as it would be in any emergency or violent disturbance etc) you are going nowhere.