At STEP3 of the online booking process (this is after you have made your vehicle choice) Towards the bottom of the page, you will see a section called EXTRAS. Here you can reserve extras such as baby seats, GPS or add Additional Drivers. The price of the extras are displayed but these are paid for on arrival, at the desk. Certain car models do not display all of the extra options, so experiment by choosing a different car model if the extra you require is not offered.
Also, if not baby seats are offered at all during the online booking process, you can always rent those at the desk