Kid's fine, the older you get, the higher a normal BUN would be. The BUN goes up with decreasing ability of the kidneys to excrete, so a lower number is not of concern. Although the overhydration issue is interesting I've never encountered a BUN that was of clinical significance because it was too low.
Remember that kids tend to have O2 sats 98-100% normally, while healthy adults max out at 96-98% just because their systems are older- this is a similar situation. A better number may be outside usual limits, but not necessarily something you need to address in a care plan.