Firstly, Splinter wouldn't get hit with the shield. He and the turtles are all bullet blockers/dodgers, so the shield is painfully slow in comparison. He'd probably catch it and fling it back with his tail.
Second, even if he did get hit; he took an extended beating from Suit Shredder and it took the lair exploding and him being buried under rubble to hurt him enough that he could die. He'd eat that hit for breakfast.
