Using a car for three from Brussels to Ghent and further to Bruges makes no sense anyway in terms of time. Train Brussels - Bruges takes 1 comfy hour and Ghent is halfway, you can't beat that with a car, especially if you are not familiar with all the ins and outs of driving there. Train ticket to Bruges costs about €15, child about €8, return dubble. You can get out in Ghent for instance spending the whole day there and continue to Bruges later with the same ticket as long it is on the date the ticket is issued.
As all major railway stations in Belgium Ghent Sint-Pieters has coin operated lockers. If nothing has changed a big one around 1,8x0,6x0,6m costs €4/24h, other lockers are half or third in height and cheaper. “De Lijn” tickets for tram or bus are the same, from the machine and counter cheaper as from the driver. If you only go up and down with three with the tram in Ghent and one way with the bus in Bruges from the station to your sleeping place, buy a 10-ride ticket for €15 at the counter.
Edit: Ghent Sint-Pieters station: they renew the platforms there, but has as far as I know no effect on travelling, but you can ask in Brussels.