child benefits affected by rental income?

hi my wife and i want to move but can't sell so thinking of renting our property out. neither of us earn over £50k but a GROSS rental income could take us above hence child benefits would go down. the net prifit we would make from rental income however would be very very small as the mortgage on it is still large. This net profit added to our salary would not take us above £50k.
so is rental come added to 'one of our salaries' which seems the determined factor? and if so is it the gross profit or net profit?
would seem unfair if its the gross as thats just a nigh on unprofitable income
thanks