CIM Motor Direct Drive or Supporting Axle

[GeeTwo]

From a mechanical standpoint, two CIMs is overkill unless you're planning to project a stream of balls. Unless your parents have 101 Dalmatians fetching those balls, you won't need this much power. If you make your wheel systems heavy enough (that is, with a sufficiently high moment of inertia), you can spin your wheel tips just a bit higher than the desired launch speed.

As ballistic range is simply v² sin²2θ / g where θ is the launch angle, to throw a ball 32 feet through the air, you only need a launch speed of 32 fps at a 45° elevation. For a CIM free spinning at 5310 rpm, that would be about a 1.4" diameter wheel. A 1000 rpm gearmotor on a 7" or 8" wheel would provide this same speed. Motors of this type have an 11Ω resistance when not turning (I have several handy, including one that actually lives here at my computer desk.) This implies a stall current of a bit over 1A. If you use one for each wheel, you're going to have about 6W of mechanical power assuming a nominal 50% efficiency at high speeds (50-80% of free). A tennis ball weighs 2.1 oz, so throwing it at 32fps requires mv²/2 = 2.1 ft-lb, or 2.84 J. At 12W mechanical power and even assuming another 50% loss due to friction, you could throw at least 1 ball per second.
For tread, I would definitely recommend something much softer than a lawn mower tire. AM solid rubber wheels would not be too bad, but blue textured nitrile would be even grippier, though it might need to be changed out every few months of dog play.
Pololu and other vendors sell decent 4mm D-shaft hubs, though getting this up to an 8" wheel might be a stretch.

I don't seriously recommend the 25m gear motor, but I wanted to point out that a pair of them was (at least theoretically) adequate to the task. A BAG motor or two with VP gearbox(es) and colsons is probably the easiest way to go with plenty of room for adjustment.