Can fifth disease be harmful to my toddler?

Not generally. Once the rash of fifth disease is present, children are no longer contagious. The rash may return over the following weeks-months with exercise or sunlight but is not worrisome. Some children may get joint pains that usually resolve in 2-4 weeks without long term complications. Other than not feeling well, it is generally not a harmful disease.
In Some Cases. Fifth's disease, an infection resulting from parvovirus b19 causing a characteristic "slapped cheeks" appearance in children, is usually a relatively minor illness. However, children with sickle cell disease and those with suppressed immune systems (from cancers, transplants and other immunodeficiencies) are at risk for more serious complications.
Not usually. However if your child has sickle cell disease or some other immunodeficiency, then they could have more serious complications. Also, if your toddler is diagnosed with fifth disease, make sure you keep them away from pregnant women, since the virus can cause miscarriage. Hope your toddler feels better soon!