The Welsh Assembly's Health, Social Care and Sport Committee will launch it with a workshop at the National Eisteddfod on Anglesey on Tuesday.
People aged 11-16 will talk to committee chairman Dr Dai Lloyd and Ynys Mon AM Rhun ap Iorwerth.
Dr Lloyd said the inquiry would look at whether "efforts to increase physical activity are working".
He said it would also consider whether girls have fewer opportunities and different attitudes to boys when it comes to exercise.
The latest figures from Sport Wales' school survey, released in 2015, showed 48% of pupils in Years 3-11 took part in extracurricular or community club sport three or more times a week - up from 40% in 2013.
At the time, there was a "gender difference in participation levels", with 52% of boys taking part three times a week versus 44% of girls.
During the launch, AMs will join some young people in pedalling 1km (0.6 miles) on an exercise bike, with Dr Lloyd - who practices part-time as a GP - on hand to take their blood pressure.
He said: "Keeping active brings long-term health benefits, which is why it is so important that children and young people in Wales make it part of their everyday lives and find a sport or activity they really enjoy.
"Active children and young people are more likely to stay active and be healthier as they grow up, helping to reduce future pressures on the NHS in Wales. The Welsh Government has previously estimated that each year, the cost of physical inactivity to Wales is Â£650m."
A consultation will be open for people to submit their own ideas and suggestions to the inquiry via the committee's web page until 15 September.
An inquiry into the barriers stopping children and young people becoming more physically active has been announced.