My Big Girl Undies and My Big Boy Undies by Karen Katz
This book comes in two editions, one for boys and one for girls. These books celebrate the fun of underwear, and also acknowledge the occasional accident. The prose is largely the same, just switching out boys and girls. Neither book is really gender specific, at least not beyond the patterns on the underwear and the gender of the children in the illustrations. In other words, your child won’t learn anything about the differences in going potty when you’re a boy or a girl, such standing up or sitting down, wiping front to back, etc. Nonetheless, this is a carefree and fun way to enjoy the potty process, particularly the celebration of wearing underwear.
We have two great resources for training boys and girls here on the blog that do help with gender specifics, and these would be a good addition for Mom and Dad if using these books.
Potty Training Boys and Girls: A mom to 11 kids talks about some of the insights she got from raising her family.
Specific Tips for Girls and Boys: A list of tips specific to training each gender.