See i would have thought that by the time you'd filtered out all the people that:
a) don't have kids the right age
b) would immediately think 'scam'
c) would ring back immediately
d) only have one child so the 'which one is this' would catch them out
e) are male (so aren't 'mum') so they'd know it was fishy
there'd be hardly anyone left to make it worth doing!
but i guess there is, and it does confused
This is kind of the point. Remember these scammers will be targeting thousands and thousands of people. They only really want responses from those who are gullible, uninformed or vulnerable enough to hand over the money. They can't handle responses from thousands of people. So it's a way of filtering out "time wasters" from their end.
It's like the Nigerian prince scam. Most people now could spot that a mile away. The ones that can't are usually the vulnerable ones who will fall for it.
The scammers will therefore get quite a high hit rate. Most will ignore them, but the ones that don't will be the ones they steal from.