An 11 or 12 year old is still a minor and usually the airline would try to have one parent sit next to the child or across the aisle or in front or behind or so. Picking both of you may have been their best option, but as you said, if so they should have been straight about it.

If you feel strongly about it, you could tell the IFM (Inflight Manager) what your experience was and express your displeasure at what appears to have been dishonesty by the check in staff (though one can't tell for sure). The IFM can create a report (called a voyage report) about it that gets send to customer affairs though it's usually primarily intended for onboard items / issues.

The other option is to give feedback via the VoC survey (voice of customer) if you receive it post flight and mark down the check in experience and explain why. That assumes you get the survey.

Lastly you can write in to SIA Customer Affairs post flight by giving feedback on your experience. I wouldn't demand anything in compensation wise, but request this to be looked into and reverted on as honesty should be a core pillar of the customer experience. Something like that.

Btw, if that really is what may have happened, the family that has your seats now may not know anything about it.

YMMV.