/I can't help wondering who the real benificiaries are, the charity, the advertising agency, the film maker or the TV company....or perhaps even the children? /
Well the charity will calculate the money generated minus all costs (media, production, agency creative and account management)
Decisions on repeating the campaign will depend on the Return on Investment. In some analysis, measurement of subsequent donations and estimates of a new donor's 'Lifetime Value' may be considered
Overall, (generalisation alert) fundraising/ charity marketing raises more for the good cause than it costs
That's why charities do it.
Splits on their spend on the cause and marketing, admin etc are available here
http://www.charitycommission.gov.uk/