Very small grommets keep splitting
If there is nothing technically wrong with the grommets then there is probably not enough material in their grip when you are attaching them. Thus they over crimp and split. 

You could try adding a washer shaped bit of cloth that matches the diameter of the ring of the grommet to add more bulk, or try not squeezing them as hard. Using a washer of extra fabric will help them last longer as there will be less stress on the t-shirt material at the grommet.