Michael Rickman died on Saturday in the burns unit at Broomfield Hospital in Chelmsford, Essex, where he was taken after the accident on 1 July.
Michael Bonfield, aged 12, who also suffered burns in the accident in Ladywood road, Darenth, is still seriously ill in the same hospital.
He is in a stable condition and has been able to leave intensive care.
The two boys, who lived locally, suffered burns when the mini-motorbike caught fire after the collision.
Michael's mother, Anita Bonfield, later urged parents whose children had mini-motorcycles to "think twice about letting them ride it or even be a passenger on it".
An amnesty is currently under way in the Shepway area of Kent to encourage people to hand in off-road vehicles such as mini motorbikes which are illegal to use on public land or roads without insurance, road tax, registration documents or number plates.
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