How to remove the “Children” or/and “Adults” field in search form?

This can be set in HBook > Search Form

1. I only need “Check-in” and “Check-out” dates, but I do not need the number of persons.

You will see an option Display a field “Adults”. If you only need the check-in and check-out dates, you will set this option to “No”. This will disable both the “Adults” and the “Children” fields for the search form.

2. I only need one field for the number of guests. I don’t need the “Children” field.

You will see an option Display a field “Children”. You will set it to “No.
You can modify the label “Adults” in HBook > Text > Search form.