Questions that must arise:
1) Did the boy, bus driver or any bystander have to offer a prayer for God to intervene? (One assumes that the nature of the accident preculdes the possibility of anyone having the time to do so, but your description of the accident is vague, so I want to be sure.)
2) Given that God has shown that he is prepared to intervene to preserve mortal life, why does he fail to do so more often? Why was this one boy and bus scenario so much more important than say, Shannon Matthews, currently missing from home? We know prayers have been offered for her too.
Why, for example, does God not intervene to save children in US high school massacres? Ok, if we allow that God has to have a prayer before he can act (which is nonsense, but we'll allow it for now), maybe the first few would die, but the others in the next classroom would surely be praying before the gunman burst into their class?
What about where death is not quick, but slow, lingering and painful? How about a children's cancer ward - this child lives, but five others die (or five live and one dies, it matters not). Are some children dying of Leukaemia more worthy of saving than others?
Why is your God so inconsistant? Is he not all powerful? Is he capricious, evil, lazy, inattentive or what? What criteria are used to determine which children live and which die?