My wife asked me what I would do if I was out and about alone with my daughter and had to use the restroom.
I told her I hold it. Then I thought- what would I do if it was an emergency (e.g. the runs)? What do you, your husband, father, or men you know do? Edit/conclusion: Thank you everyone for your responses, the overwhelming support is nice. My wife has conceded to support the mass opinion of just bringing her to a stall in the men’s or unisex bathroom. Turns out the question came up when she found out my father-in-law was having her stand outside the porter potty in a busy public park parking lot while he did his business every time he was bringing her to the park alone. 
We decided to not go alone or let my in-laws go alone with our baby/toddler to where there is only porter potties, and stick to places with actual bathrooms.Again thanks everyone 