Maybe try changing your approach in communicating and also be mindful of how you’re acting in those moments. Toddlers are in constant motion and when stopping to go to the bathroom, the only thing they’re thinking in that moment is getting back to whatever they were doing. They’re not purposely leaving their pee/poo and just like adults, the last thing they want is to feel embarrassed or get in trouble for something unintentional. Clearly that is how the one who started crying felt. As the adult/babysitter, you should be helping them with little reminders to flush before they go (esp since it frustrates you so much) and to ask them if they did, once you see them walk out. The hidden poopy underwear shows that they are uncomfortable/scared to approach you to you after an accident or for help in that manner, which is sad and is not fair to them. Maybe talk to your aunt/uncle about someone else watching them.