Welcome to the question forum! Thank you for your question.
If your BGL is above 15mmol it’s really important to test for ketones – either by urine with a ketodiastix or if you have an Optium meter, you can test you blood ketones. If you have ketones or are you are unwell then it’s not advisable to play sport. The presence of ketones indicates that you need more insulin – discuss this with your diabetes team.
If your BGL is over 15mmol, with no ketones and your not unwell, it’s OK to do some low or moderate intensity exercise (eg. walking, tennis) but not strenuous exercise like a game of competitive football or basketball. It’s a good idea to check your BGL again in 1-2 hours to see if it’s come down.
A book called Caring for Diabetes in Children and Adolescents – a parent’s manual has a great chapter on exercise and sports. It’s available from DA-NSW and the Children’s Hospital at Westmead or check out Living with Diabetes in the Parents and Carers section of this website.