Are your themes child theme compatible?
Yes, 100%. Also all ThemeTrail themes come with a child theme to get your customizations started in no time.
When to use the child theme
Never edit any of the theme files, period!
This is the main purpose of using the child theme: the possibility to customize your theme files without having to touch any of the parent theme files. Becasue when updating the theme, all changes you have made to the parent theme will be lost, and this is where the use of the child theme is required.
Custom CSS and JavaScript can be added through the Theme Options Panel in your WP dashboard. No need to use the child theme here.
To learn more about child themes, visit: http://codex.wordpress.org/Child_Themes