Welcome to the forum.

I'm not a doctor but I think your GP is concerned that this may be a bone sarcoma.  I've checked the NICE guidelines, and normally an X-Ray is only used when the patient is a child or young adult.  If this is one of your children I can only imagine the fear that's going through you now.

The next stage is an urgent referral to a specialist consultant service.  The GP will arrange this and the hospital will probably phone you to make an appointment.  The consultant will arrange follow-up tests, which may include MRI, CT, ultrasound and biopsy. 

Today, GPs are trained to refer to a specialist service if there is any suspicion of cancer, and as a result most patients who get referred for any cancer turn out not to have it. Generally speaking, the figure that's generally quoted is that 9 out of 10 urgent referrals turn out not to have cancer.  I don't know the particular figure that applies to bone cancer, but I would urge you not to assume that a cancer diagnosis is certain even if you get an urgent referral.

At this stage, you might be tempted to start Googling.  If you do, please stick to UK sites only. Avoid American sites - they're often very alarmist and frightening.  I suggest you stick to Cancer Research, the NHS, and the charity Sarcoma UK.