My son is four and loves to do things by himself, including washing up in the tub. I found the perfect soap for him to use that was made especially for independent toddlers learning to do things for themselves. It's the Johnson's Buddies Easy-grip sudzing bar. The cost is very reasonable at only .99. The Johnson's Buddies soap is just the right size for toddlers little hands, and is enclosed in a pouch that makes it easier for him to hold without being slippery. It lathers up nicely with a nice clean scent.
The only thing I have found that may be a problem is that he sometimes tries to get the soap out of the pouch. The bar is small, so it doesn't last a real long time, especially if he leaves it in the water, but it does make him want to wash and helps him do a good job.