Still can book as adults.............. if not sitting together then it is at your risk.
No, the answer from the chat team is children they must be booked as a child or could be denied boarding. This is also suggestion in paragraph 6.2 of the T&Cs.
Free to take your chances at checkin. So should Ryanair hold standard seats just in case someone makes a late booking for a family ?
No, but if someone is forced into a supposedly free seat reservation (for the child), available seats should be given at the same conditions if standard ones are not available.