Buy to Let, with dependants as tennants

Looking for some advice.
Do I need a buy to let mortgauge on a house I have a residential mortgauge on already, if Im renting it to my ex with my child living there as a dependant?
Im not looking to make any profit, but how will this affect any tax I pay?
Also I dont want to get stuck with a big bill if the central heating leaks or something. Will all this be covered with landlords insurance?