That is a deplorable tactic. Someone has posted in news I think about a business man who has successfully invoiced one of these direct marketing companies for unpaid invoices - he told them he would bill them for his wasted time - but this is not a convenient option.
We have the TPS - but that only works for UK registered companies and callers - Hence the number of calls from DM companies based in India.
And there is also the problem of who you take your complaint to - Your service provider, OFTEL the government regulator, the company themselves, although often you cannot get contact details to direct your complaint.
Added to the problem is that for most people automatically recording incoming calls for evidence in support of a complaint is not possible and it gets even harder.
Targeting kids really is unethical behaviour though, and should be complained about if at all possible. Maybe the ASA?