I find it hard to believe that trained survey companies are cold calling children and asking parents for information about how many kids of certain ages there are in each household. Only children over 17 get declared; there is a reason for this- child protection.
If they are doing this really needs reporting to the Council as it is putting children at risk- no strangers should be asking about childrens details and asking to speak directly to them and parents should not be giving out such information.

The best way of soliciting childrens views is surely to visit the local schools to speak to local children in a supervised environment.