I hope your little one is ok now, your question about what to look out for when they are poorly is a good one & can be more important than numbers on a thermometer.
The main things are how they are acting - major red flag symptoms are not wanting to do normal things, really floppy, lethargic etc -
another thing to look for is taking less liquid than normal about 50% less than normal would be a real concern.
Taking less food is less of a concern.
A temperature is something to note, but you will see on MN that some people have children with temps of 40 and they are fine & others it may be 38 and they have other red flags, so go on how they are rather than just numbers.
Other things to look for are how if they are working hard to breath, or fast breathing.
The above are general points, in regards to COVID, the three main symptoms are what trigger a test in an adult so high temperature, loss of smell/taste & new & continuous cough.
For general health there was a lullaby trust baby health app that was always very good to work through.
For Covid the symptom tracker app, not the test and trace one, but the research study one may help reassure you & you can report for a child.
My sons consultant said to think about normal childhood illnesses above COVID & if you are worried at all make contact with a Health professional.