Thank you Jeremy and Alyssa :P

You can sound more casual if you say "I washed my hands before eating dinner." An example of "wash up", "washing up" or "washed up" would be: "wash up" "I will wash up before I take my medicine." (This is used for future tense) "washing up" "I'm washing up to protect myself from germs." (Used for present tense) "washed up" "I washed up before I start my day." (Used for past tense) Those words my vary from time to time depending on how they are used in a sentence structure. Hope this helps.

Example: “Kids, it’s almost time for dinner. Go wash up!” I don’t use this phrasal verb often (or ever?), so I’m no expert, but I believe it is implied that the hands will be washed when this expression is used. Also, if the child’s face is dirty as well, a parent may expect the child to wash the face and hands.