Widget fields customisation?

Hello,
probably you are selected the showing of the “availability calendar” and not the “booking form” with calendar at the Booking Calendar widget configuration section. Please recheck it.
Kind regards.

I have the booking form selected… I see the widget with the default fields on the front-end, but I don’t understand how for example to add the “child number” field there…

Hello.
you can customize the booking form feilds at the any paid version of the booking calendar at the Booking > Settings > Fields page at top form.
Please read the Booking Calendar version overview here
You can test Booking Calendar live demos at this page to be sure in functionality and do not have misunderstanding.

Thank you. I have some other questions:
Can I end up (with only some CSS tweaking and field customization) having a booking form like this one (with the start and end date like those you see there)? I also have rooms as articles in my wordpress database. I also have a product listing system that is built in a theme. Is there a easy way to connect your plugin to this system?

Thank you,
NM

Hello.
Unfortunately its not possible. Using the CSS, you can only customize the style of the fields but you can not add new fields.
Also, if you are need to have the search possibility as at that site, you are need to check the Business Large version. Please read the Booking Calendar version overview here. You can test Booking Calendar live demos at this page.

But can you add/edit fields using html/php easily?

Hello.
It’s possible, but it’s not so easy. Because its require the modifications of files in several places (showing the booking form fields, then saving it in specific format, and finally showing of these new fields) and then full reinstall if plugin. So more easy way, it’s to use Personal or any other payed versions.
Please check it at the live demos at the plugin site.