I was shocked when a child came over for a play date and peed in my Nanny Family’s backyard bushes! He told me his nanny always brings toilet paper or a napkin with them in case he has to go potty behind a tree at the local playground. There are portable outhouses and a bathroom facility at that county park! Of course there are some parks and trails where bathrooms may not be available but it is terribly unsanitary and unnecessary to urinate in a backyard or in a busy park.
I found these Portable Urinals for emergencies when out with kids and Portable Emergency Toilets for long car trips or camping which just seem like cleaner, more sanitary options to me. There are Portable Urinals for boys and Portable Cup Urinals for girls. They do not spill when tipped over. These sturdy Disposable Bags are great for vomit or urinating as well.
If you ever allow your Nanny Kids to pee behind a tree or Bush please let them know it’s absolutely inappropriate to do so at someone’s house, yard, or a park with toilets.
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bethebestnanny.com 
I used to teach preK in a public school. One of my students urinated on the side of a portable classroom at recess! We had to have a talk about that!