What is the ruling on mixing the money of an orphan under our care with our own money, from which we spend on the the household, and is it permissible for his money to be more than what we spend on him?

There is no objection to taking charity to the extent of one's expenses or less. Whatever exceeds that must be preserved for the person for whom the charity was intended. You will receive an immense reward for the fostering and kindness you show.