The guy wouldn’t be able to answer you because the club don’t have a policy on how it is possible to ever add a child season ticket to accompany an existing adult one, unless the seat next to you happens to be available and you manage to grab it during a window for new sales.
Your only hope is to relocate your own ticket(s) to a spot where there is a spare seat next door. Be waiting with a valid match day membership for the child for an announcement that any new season tickets are being made available. Then if all that happens and you’re lucky enough to get as far as being offered one, hope that seat is still available.
Alternatively, the online system isn’t, or at least wasn’t, set up to enforce the rule about children sat alone. So if you can grab one for them anywhere, do that and make do for a year and look to relocate the following season.