Child benefit is not means tested so it won't change however much the earnings are. Child benefit is �17.45 per week for the first child and �11.70 per week for each of the other children. So �40.85 per week is 'safe' however much is earned.
Child tax credit is calculated at �1765 per child per year plus �545 per year 'family element'. That works out at �112.30 per week. These figures apply as long as the annual income doesn't exceed �14155 (�272.21 per week).
Anyone can earn up to �5035 per year (�96.83 per week) without paying income tax. The next �2150 per year (�41.35 per week) is only taxed at 10p in the pound, so someone earning �135 per week only pays �3.82 in tax. Beyond this aggregate point (�138.18 per week), the tax man takes 22p from every pound earned.
It should be remembered though, that National Insurance also cuts in above �97 per week. For every pound earned above this figure, the tax man takes 11p in National Insurance (as well as the income tax referred to above).
Chris