I do epidemiology, work with a virologist...
From my primitive understanding, the more people around, the more exposure to each other, the more chances viruses have to evolve (viruses love to mutate). So the more likely they will come up with a mutation which makes them a bit more ... virulent :). So definitely viruses are in a race against 'us' to get us.
What OP describes is one child presenting with multiple viruses. But do we know this didn't happen a lot in past? We don't, actually. As head of infection control said to me not long back, if the hospital test a very ill person for virus X that matches symptoms, and find virus X, they don't continue testing for other viruses. They just assume virus X is the problem & treat accordingly. They don't keep looking for all the bugs unless person doesn't respond to treatment for virus X. Or develops symptoms that don't fit with virus X.
Odds are, treatment for virus X will also help deal with viruses Y, Z, A, B & C that person may have. So no one will ever know about YZABC.
Kids are super spreaders & get multiple things easily (due to lack of prior immunity). We should expect them to have multiple things to fend off typically.
So to me, OP's story doesn't mean there are more viruses.. it means there is More Testing! I wonder where this is, where they started to do more surveillance.