I don't know if they still make it....but....
MICKEY MOUSE UNDERWEAR!!!
As a great nanny once said, "in every job which must be done, there is a element of fun". I had the Mickey underwear as a kid when potty training was going on. Yes, I do remember that far back.
As much as I hate using extrinsic rewards with kids in the classroom, it pays off big time for potty training. My mom is a pediatric nurse practitioner and developed a system which has been successful with many, many children.
Urination
Set up a sticker chart. Get a piece of poster board and divide it into seven sections for each day of the week. Label it "My Big Boy Chart" (or "Big Girl" if you have a girl). Take your child to any store which sells stickers and pick some out that THEY like. If your child successfully urinates, they get to pick a sticker and put it on the day of the week. This is also a good way to teach sight words with days of the week (it's NEVER too young to start encouraging this).
Bowel Movements
These are trickier, so in addition to a sticker on the chart, reward a successful bowel movement with an M&M. You will probably have to help your child wipe, but as long as they are able to tell you they are ready for it, it should work fine until they are neat enough to understand how to do it properly.
DO take the pull-ups, and get the waterproof ones for pool areas since it is harder for kids to know when to go in the pool. Also, don't use the underwear at night if they're not fully potty trained. However, using this system, you can potty train rather quickly since the rewards are there.