Agree with all @sultanasofa's points. This is an abysmally designed study, abysmally written up.
A couple of things not already mentioned that leapt out to me:
They were comparing bone density to baseline, i.e. each individual's bone density pre- and post-treatment. That is bizarre when you are studying adolescent children. Their bone density should be increasing, so the fact that it didn't decrease during treatment isn't reassuring at all - quite the opposite. Where are the comparisons with a non-treated group? Even without a formal control group, they could easily have compared the treated children's bone density to average for their age group. The fact that they didn't suggests they are trying to gloss over the very worrying finding that PBs inhibit normal skeletal development.
One of the authors is Russell Viner. Guess what his job is now? President of the Royal College of Paediatrics and Child Health 