Xpath query to find elements which contain a certain descendant

"has a descendant named interestintag" is spelled .//interestintag in XPath, so the expression you are looking for is:

//table[@name='important']/tr[.//interestingtag]

Actually, you need to look for a descendant, not a child:

//table[@name='important']/tr[descendant::interestingtag]