My daughter also has had a lot of issues with nausea. In her case, we kept blaming the Crohn's - inflammation in her stomach. Eventually, we did an endoscopy which showed only mild inflammation, so then her GI ordered a gastric emptying test.
The gastric emptying test showed delayed gastric emptying - and she was diagnosed with gastroparesis. It is a fairly easy test - your child has to eat a small meal - usually eggs and toast - that is mixed with a small amount of radioactive tracer. Then they take pictures over four hours to see how it empties out of the stomach.
If there is more than 10% of the meal left in the stomach after 4 hours, then you have a dx of gastroparesis.
In her case, she has been on medications to speed up motility and we have had to change her diet. Fat and fiber are hard to digest so she avoids those. Liquids are also easier to digest than solids.
It is definitely worth asking about - I wish I had asked much earlier to do this test. In my daughter's case, her GI believes her Gastroparesis is independent of her Crohn's but of course, we can't be sure.
Good luck!