To add tags to children you would go to the Family tab > Family List > Select at least one child you wish to add a tag to > Click the top drop down (3 dots) > Add Tags. From here, click Add/Edit Tags > New Tag > Add in tag name > Click Save. 

You can also choose to categorize the tags by clicking the drop down where it says Uncategorized > Add/Edit… > Add Category > Add in category name > Click Save. Make sure if you do create a category that you select if from the drop down. Select the checkbox beside the tag to add the tag to the child(ren) and click Save.