If you have reserved seats as part of your booking then there is no benefit to checking in 30 days in advance versus 2½ hours, other than to have got that small chore done. That said, it doesn't seem to affect the cost of making changes, but it certainly can't make things easier if you've checked in (apart from anything else, you will need to check in again if you change something!), so I wouldn't bother for now.
In terms of what changes you can make: Unless you bought Flexi-Plus tickets your only option is to "rebook" (change to another flight), which costs £45 per person. You cannot do this in two stages (i.e., cancel the flight for credit and then rebook later); you have to say "I was travelling on the 18th but now I want to move my flight to the 24th" all in one go, with your new choice of flight ready.
If the original flight was £50 and the new one is £75 you will also have to pay the difference. If the new flight is cheaper you don't get the difference deducted from the rebooking fee, of course. (Ryanair is a plc and you can buy shares, which doesn't get you any privileges but might at least make you feel slightly more favourable about their rather rapacious business model.)
I haven't been able to find out if you can rebook for only some of the passengers on a booking, as opposed to all of them. You can maybe try this yourself, though. Click on the button to change the booking and see if it will let you select individual passengers. Don't worry, you can back out before paying so that the change doesn't become definitive!
Presumably you bought travel insurance, but whether it will cover cancellation "to spend more time with a sick parent" (as opposed to something a bit more objective, like a death) will require close study of the T&Cs.
Our kids are late primary age so it’s not the end of the world if we aren’t sat together on the short flight.
It might not be the end of the world, but Ryanair expects children under 12 to be sat next to an adult, and requires you to purchase a seat for the adult to allow this to happen. They then give the children assigned seats for free.