Working tax credits are a good idea in principle. Low pay is a big problem, and shifting the welfare system away from being a safety net towards topping up the incomes of low-skilled people who are in work is probably the right approach.
It doesn’t make sense to both tax people and pay them benefits. Cutting income tax and, especially, raising the National Insurance threshold on low-income workers is less complicated than making them apply for tax credits, and probably would incentivise work by getting rid of the tax credit withdrawal 'tax', without removing their incentive to join the work force (as ditching tax credits alone might do).
That isn’t what’s happening here, though. These cuts are meant to reduce the deficit, so they won’t be offset entirely by tax cuts. That might disincentivise work (reducing people’s incentive to enter the work force) and will clearly make some poor people worse off.
Lowering the child tax credits threshold and increasing the withdrawal rate would be one of the least harmful ways to cut tax credits, because these are not tied to work and because they are paid to couples earning up to £41,000/year, which is quite high.
Housing benefit and pensions would probably be better things to cut. The £26bn housing benefit bill could be reduced significantly by reforming planning to allow more houses to be built. Abolishing the pensions triple-lock and increasing pensions in line with inflation only would produce major year-on-year savings – this year, the £92 billion pensions budget would be essentially frozen.
Deregulations that cut the cost of living would offset some of these cuts. Housing and, for people with children, childcare are the biggest costs for people on low incomes, and payments for childcare in particular are built in to the tax credits system. The UK has some of the harshest regulations in Europe on both of these things, driving up costs. If the government made it easier for the private sector to build more houses and relaxed regulations about staff:child ratios in crèches for children, the cost of living for poor people would fall significantly.