V.L.
Definitely go with sitting down. They can be relaxed and focus on the function itself. The standing up thing requires aim, which is a different skill, and will be quite a challenge for you to keep your bathrooms clean! That said, I have heard people put a cheerio or something in the bowl for them to aim at when they are ready for standing. My son began peeing standing up during the summer when he could be outside peeing on a specific bush, to get the hang of it. Good Luck!