Contact Form 7 - Multiple text fields on the same line

You can simply use HTML table structure!!!

<table>
  <tr>
    <td colspan="2">[text* Name placeholder "Your Name"]</td>
  </tr>
  <tr>
    <td>[text* Class placeholder "Enter Class"]
    </td>
    <td>
      [text* Medium placeholder "Enter Medium"]
    </td>
  </tr>
</table>
[submit "Submit"]

You probably have a CSS rule that makes the <p> element full-width.

Why not use <div> and use the theme's classes?

<div class="form-row">
   <div class="grid-33">Number of adults [text your-adults]</div>
   <div class="grid-33">Number of children under 6 years old [text your-little-children]</div >
   <div class="grid-33">Number of children under 12 years old [text your-big-children]</div>
</div>