I do not see the issue with this. They shower together to get clean. There is nothing sexual about it. I think this can help kids as they grow up to see there is nothing wrong with showering or bathing together. Lets them see that you can be comfortable in your own skin and not have to hide all the time.
I can see showering with your children when they are very young. I did. But as they get older I wouldn't do it. That said, I don't think it's an issue until kids reach puberty. We need to stop interfering with every aspect of parents raising children and concentrate on those who truly hurt their kids.
Yes while they are growing it is not that big of deal as it saves time and allows you to make sure they are clean. As they get older they naturally stop wanting to bathe or shower with you and you let them go their own way. This system helps you raise better kids in my book.