If your wife is employed then she may not be eligible for seeking maintenance.
In that case the court may order to pay maintenance to your child.
Th subject of granting maintenance will depend on how effectively the claimant proves her case and how intelligently the respondent defends himself.
She has to prove your income while claiming maintenance with documentary evidence supporting her claim
There is no parameter to measure the quantum of maintenance hence it will depend purely on the discretion of judge.