Little straining on urination is not a problem.
If she statins abnormally, there might be associated constipation. Hard stool might press the urine pathway and she might strain. Sometimes the skin at the Urinary opening might stick to each other (vulval synechiae) which needs to be released and cleaned.
Next Steps
If you are not able to see the Urinary and vaginal openings separately because of stuck skin, you need to consult a doctor, preferably a paediatric surgeon.
If opening is normal, just increase the fluid in her diet. Reduce artificial powders. Introduce natural diet which are smashed to semisolid which she can have easily. All these things will avoid constipation.
If symptoms persist, you need to give urine tests to look for infection and cause of infection.?