These shorties are so adorable on my son; however, they're very slim fit. Unlike the photo which shows some gap around the legs, his hug right around his thighs. He wears a 3T in pants, and I purchased 3T here. There is no room to grow, and I suspect he'll be out of them before the end of summer. He wore them without a swimming pull up. I don't think there would be room for that. That said, they're really cute and he seems to like wearing them, so I'm not complaining! I like the more fitted look and material of these shorts over the long and baggy swimming trunks, and I would definitely order another pair, but likely would choose a size up next time.
Overall, would recommend, but if you're between sizes or have a studier/chunkier toddler, maybe go up one size.