Well there is a definite issue in this circumstance - a 4 year old definitely should not be seated apart from both parents if all are travelling. Apart from anything at that age they really do need looking after.

But even if a family does end up in that situation, whatever the rights and wrongs for ending up there, they shouldn't be so presumptive that the stranger will simply move seats for them. A definite failure by check-in staff in this circumstance. Or if they didn't use airport check-in, then definitely one for cabin crew to sort out.