From the FR website
Each passenger is permitted to check in up to 3 bags with a maximum combined weight of 15kgs, subject to the payment of the applicable checked baggage fees.
Any passenger exceeding their 15kg personal checked baggage allowance will be charged for the excess at the applicable rate per kilo.
There is no checked/hand baggage allowance for infants. However, one fully collapsible pushchair per child may be carried free of charge. Additional infant equipment such as car/booster seats and travel cots may be carried in addition to the personal checked baggage allowance of 15kilos per person. Infant equipment (car/booster seats and travel cots) are charged at a separate fee per item/per one way flight
If you had taken the time to read the terms and conditions associated with your ticket you would have been aware of the charges.