She's not too young to have mono, but this is usually associated with other symptoms, such as high fever. If the cheek redness suddenly started, she may have a different viral infection ...something known as "fifth disease." There's no need to be alarmed however, as this is an extremely common childhood illness, and usually, very mild. The most noticeable symptom is the red cheeks, almost looking as though she's been slapped, and it may come and go, getting worse when she's hot, stressed, or ill . In addition to the rash, cold-like symptoms are common, so it's not surprising that she seems to have recently had some cold symptoms at the same time. Sometimes, the rash can seem to spread to the child's trunk (chest and back), hands and feet, but many children have the virus with few to no symptoms at all. Usually, the cold-like symptoms begin before the rash, and at this time, they are contagious. Once the rash appears, however, this is no longer so.
Like most viral infections, there's not much you can do for it, as antibiotics don't work for viral infections. Fifth disease is treated the same as a cold, with rest and fluids, and perhaps tylenol or ibuprofen if she's uncomfortable, achy, or develops a fever. The rash usually lasts from 7-10 days, but can persist for up to a month in some cases.
If the rash or cold symptoms persist longer than 2 weeks, or he has a persistant fever or one higher than 103.5 degrees, or begins to develop other symptoms such as trouble breathing, sore throat, or blisters on the skin, you will need to have her seen by her pediatrician to rule out other possibilities, including other infections (roseola, scarlet fever, rubella, and measles), allergic reactions, or skin conditions such as eczema or rosecea.
I hope this helps!