I think it would be very unusual for a young child to use the Irish form of a surname outside the school system if the parents didn’t use it too.
Within the school system the Irish form may well be consistently used, especially in Gaelscoileanna, which are Irish-medium schools. Some of these schools use the Irish form of first names too, but others don’t tend to translate first names.
As a teen or adult, the child may themselves decide to use the Irish form if they have a particular interest in the language. The names are generally considered interchangeable. For example, a friend uses the English form of his name among family and old friends, but the Irish form (first name and surname) in other circles (work, hobby). He is a teacher and this is quite normal.
Legally (passport, driving licence etc) you can use either form of the name, but you do have to follow certain processes. You can’t just randomly decide to use them interchangeably, legally.
As an aside, Sheehan is usually spelt Ó Síocháin now.