Both child tax credit and working tax credit are means tested or income related benefits, CTC replaces support for children within the tax system and WTC replaces working families tax credit. You also mention the “Council” they would only be paying housing and council tax benefit.
CTC & WTC have no system of capital limits only interest from savings will be taken into account. Income for tax credits are based on income tax legislation, the principle is that usually all taxable income will be taken into account and other income will be ignored.
The best advice would be to declare your maintenance payment, which will be ignored by tax credits, but you have declared them.