In answer to that, the main policy involving tax credits is a major labour market intervention. It essentially favours adults with children relative to adults without. Also, arguably one of its criticisms is the subsidy to employers as well as employees.
One thing we notice is that while the number of children taken out of poverty has gone up quite substantially, the number of children who seem to need support from the state to escape poverty has also gone up. To sum it up in a sentence, I think the difficulty is that after a while the policy starts to undermine itself. It's a good tactic, but it's not adequate as a strategy.