It seems like a large-scale event won't work out...
I'm (almost 100%) arriving in the morning on the 24th and leaving in the evening on the 27th. There's time for a couple of games and some sightseeing.
Yes, I'll be traveling through Moscow and will be there on the evening of the 23rd to walk around with my son. If anyone from Moscow wants to meet up. I would also be grateful to locals for advice on what, besides Red Square, might interest a 5-year-old boy in the capital.

The zoo (metro stations Krasnopresnenskaya on the Circle Line or Barrikadnaya on the Tagansko-Krasnopresnenskaya Line) is probably the best option. By the way, opposite (right near the Krasnopresnenskaya metro station) is the Barrikady children's cinema, as well as the Kinotsentr on Krasnaya Presnya.

Another traditional, but less interesting place is the Central Park of Culture and Recreation (CPKiO), metro stations Park Kultury, Oktyabrskaya. In general, it doesn't have much originality; the most interesting things are located quite far from the entrance...