My child is 12 years old. He has done a  creatinine  blood test and got a low result = 0.45. Is this of any significance? Thank you

He's healthy. As mentioned, creatinine is a substance the kidneys are supposed to filter, it's a byproduct and should be very low in the blood stream. Normal values are generally based on an average for what adults would have, and children's kidneys are healthy and much more efficient. For lab tests in children, ignore the "normals" and ask your pediatrician for an interpretation.
Rethink value. The result you quote is not "low", it is normal. Creatinine is a byproduct of normal metabolism and is removed from the blood by the kidneys. A poorly working set of kidneys may loose their ability to remove the excess, so the level goes up. When the level is below a given level (varies with age ) it is normal & not a sign of some problem.Taking the trash out early is ok!