Fever is a defense what body puts against an infection. At rising temperatures bacteria or viruses stop multiplying. A typical viral illness can last for a week to 10 days. As long your child is playful and active in-between the spikes of fever, there isn't a concern.
Next Steps
Low Hb % could be nutritional and may require Iron supplements at a latter date.
Health Tips
Lower WBC counts can happen during viral illness.