Not small buttons. Obviously small buttons require precision.
My wife's brother has muscular dystrophy so I'm familiar with motor control problems that make a touchscreen hard to use.
The reason mice or dragging a finger work well for people with normal motor skills with user interfaces is that you can make large movements quickly and can be precise. Software smarts allow for acceleration (speed of mouse/hand movement determines speed of adjustment.
if you don't have fine motor skills, there's a problem because:
(a) you'll make large, jerky movements
(b) you can't be exact in where you're pressing
(c) instead of just pressing you'll tend to press and drag
(d) double-tapping/clicking is hard thing to do
Realistically, you'd need a different physical interface that allows simple, imprecise actions to make adjustments. That would probably be a "button" interface. On a touchscreen, a button is just an area that treats each touch as a press. There's no dragging, just pressing.
But the problems with buttons (as you'll be familiar with from various user interfaces) is that it can take lots of presses to select what you want.
In this case you have a blob you want to move around. With normal motor skills, you'd move the blob quickly to the rough area you want, and then make small adjustment to get it exactly where you want it. With poor motor skills, you can't do the precise adjustment yourself, so you need the interface to help.
So:
1) buttons for up/down/left/right (like a cable/satellite remote) to replace the dragging motion
2) buttons large enough and/or separated enough that you can hit the right button each time
3) button to toggle between rough and precise movement: rough first so that each press move the blobs a larger amount, r, then switch to precise mode to get it just right; that reduces the total number of presses required.
