This sounds like what you've said before, that if you cannot think of a good reason for permitting this suffering, there must be none. But we know God is much greater than us in all aspects, so it serves that he would have a good reason that we cannot understand.
But there is also the fact that God takes no pleasure in the death of anyone, and God so loved the world he gave his only son to die for our sins so we may have everlasting life. God plans to end all suffering, and pay back those who do evil. This isn't what a sadist would do.
Romans 8:18 I consider that our present sufferings are not worth comparing with the glory that will be revealed in us. 19 For the creation waits in eager expectation for the children of God to be revealed.
To go back to your child hit by a car example, the suffering this child endures is so brief and worthless against the glory they will receive in the kingdom of heaven (which Jesus says all children will receive), that it's not even worth comparing. It will be that awesome, that no amount of suffering in this world can diminish its glory.
So we don't know the exact reasons why God allows suffering. But we know He loves us, is planning to end our suffering and destroy evil, and that our eternal life with God will be so great this suffering will be like a distant memory. Because of this, we can conclude God is not evil or sadist.