This situation depends on a variety of issues.
First off, testimony from a child has been demonstrated over the years to be unreliable. While children don't intend to lie, they seem to fabricate or over dramatize their accounts.
I doubt that testimony alone with not corroborating evidence from a child will result in a conviction. But, consider these issues:
How old is the child?
How much access does this friend have to the child?
Have the guardians of the child filed charges? Will they?
How detailed is the account from the child?
There are still many more issues here, and I'm by no means a legal expert, but one account from a child (especially a young child) should not result in a conviction. However, please understand that this account may open the door for further investigation and collection of evidence. Any prosecutor worth their salt will investigate as much as they can before filing for an indictment.