Adding a second service to a booking

So I’m hoping to use this plugin to allow people to purchase slots at a local soft play for kids but at the moment I cant seem to work out how to let someone select say 2 x child over 1 and then 2 additional children before confirming their booking? If im going to upgrade to access the payment integration i need to know this is possible first! any help would be appreciated!

The page I need help with: [log in to see the link]