I've only made 8 t-shirts so far but by seeing the way they bond onto the fabric I'd be surprised if they worked on ribbed. Maybe it would work for a one-time wear but I can't imagine it would last after being laundered. Hopefully someone with more experience will address this.
Have you checked jiffyshirts.com for buying toddler shirts? That is where I got DDs (size 24 mos & 2T), their prices are very reasonable (a few dollars a shirt + $6 shipping for the entire order unless you spend alot, then shipping is less) but they arrived fast & it was worth not running all over town looking for shirts. Even the toddler shirts had lots of color choices.
Then I placed a second order for the adult tees since they had 100% cotton Hanes beefy-t's for cheap, they are sooo much nicer than the 50/50 ones I got at the craft store (I ended up returning those) & the delivery took only 1 day.Good Luck!