The easiest way to keep track of the responses of different categories of invitees (such as Adults and Kids) is through the use of 'Tags'. Tags can be used very effectively, especially when the host is trying to collect responses from one group of invitees without the other invitees being aware of the question.
Step 1: Make sure your invitees have been tagged
Before utilizing tags to only show a custom question or meal preferences block to select guests, the host should first make sure that their invitees have been tagged in the first place. This check can be performed via the Invite List section of the main menu.
Step 2: Set the visibility of the block to invitees tagged with 'tag (Adult or Kid)'
In order for the form block (custom question or meal preferences) to be visible to one group and not visible to another, a simple change must be made in the settings of the block itself. First, you need to enter Setup > Form Builder. After that, you need to find the block you wish to make visible only to one group and click on the gear icon which opens its settings. Then all you need to do is scroll down to the 'Additional criteria' section and click on the 'is tagged with...' to add a tag, after which the block will only be visible to invitees carrying that tag.
After the setting from the example has been saved, any invitee tagged differently from the selected tag will not be shown the question while RSVPing.
Note: If your event was published prior to making these updates, you will have to republish it in order for the changes to go live.