Little kids are prone to getting sick - it's just a fact of life. Some even consider it a natural-born defense mechanism for children to expose themselves to as many germs as possible, which leads to them geting sick, in order to develop a strong immune system. The issue is when an illness lasts for too long or the symptoms become more extreme. While it may not make sense to run to the doctor every time your child sneezes or scrapes their knee, there are certainly times when a visit the the doctor is appropriate, and even advised. To best determine when you should take your child to the doctor, keep open communication with your child, if possible, about how they feel and whether their symptoms are getting worse or better. For children who cannot yet communicate how they feel, keep an eye on things like fevers, vomiting, listlessness, and alertness to help you determine whether it's time to take the little one in for a visit. Children who can communicate with parents are usually good at being brutally honest about how they feel, and will usually have no issue telling a parent or caregiver exactly what hurts, and where.
Talk to your child often about how they are feeling when they have a cold or flu, and do not just assume things. Just because you know that your child has a high temperature does not mean you fully understand the extent to the illness. They may have an especially sore throat, trouble breathing, extreme stomach pain, dizziness, drowsiness, or any other number of symptoms that are not immediately obvious just by measuring a temperature on a thermometer. Once you have found out how they are feeling, determine if it sounds severe enough to see a doctor. Put yourself in their shoes and think, if you had a churning stomach and a migraine that was blacking out your vision, would you want to see a doctor? The answer will not always be 'yes' but it serves as a good gauge.
Some general guidelines may also help you determine whether you should take your child to the doctor, or just wait it out. For example, any child that has been suffering from cold or flu symptoms for more than a week or two, depending on age, probably needs to see a doctor. While chances are small, it is wise to make sure their symptoms are not a sign of a more serious illness, such as pneumonia or strep throat. Some children are also susceptible to certain viruses, and their bodies may not be equipped to fight them off effectively, and those children should see a doctor at the first sign of illness. But, if your child has a cold that lasts just a couple of days and seems to improve with time, a visit to the doctor is probably not necessary. However, it is always a good idea to err on the side of caution. Better to make the trip and be told it's nothing, than to stay home and have something bad happen that could have been prevented.
Last, but hardly least, trust your gut. As a parent, you know your child better than anyone. If something feels 'off' to you, even if your child only has a runny nose and seems overly-tired, and you think a trip to the doctor is in order, then go. More often than not, intuition is correct. Don't discount your instincts when it comes to caring for your child.