Xavier and Anthony Gregory busked over Christmas and used the donations to buy their bikes, which were stolen last weekend.
Anthony, 9, has played the B flat cornet player for five years, and Xavier, 11, has played the violin for five years and the baritone cornet for three.
Their mother Antonia Gregory said in the past two weeks, four bikes paid for and owned by the boys had been stolen, which she described as "mean".
"You don't expect people to help themselves."
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The first two bikes were stolen two weekends ago.
Those older bikes were placed against a fence inside the Gonville Croquet Club grounds in view of the public, when someone stole them.
Last weekend the boys thought they had their newer bikes well hidden in grass under the Cobham Bridge while they went for a walk.
"Someone must have watched them," Mrs Gregory said, because when they returned, the bikes were gone.
The boys had not locked their bikes. "It would have been better to lock the bikes in public view," Mrs Gregory said.