I don't think kids would want to play in the dark and cold, knowing it's underground, but you never know . Nowadays, there's not much left from those unofficial entries (houses demolished, entrances closed down with concrete slabs, etc due to safety regulations). In 1920s these entries were used to dig chalk and trade it, hence it's called 'the labyrinth' There's a documented story, that in 1940s whole old town main carriage collapsed over 10meters down once a heavy army truck passed over it. There are quite few stories told by elderly, that many people gone missing due to home-made tunnel entries hence local government banned making them (huge fines that made chalk trade unprofitable). There is 1 official entry, where visitors can book a tour with a guide, with special branch that ends up in a restaurant . There is a plan to open up a new (old but not used for ages) entrance in different place, just to get the tour more interesting and probably longer.
Also, some historical sources say that these tunnels can go as far as to Zamość (City in ProMods), and Stołpie (ca. 8-10km from Chełm, where mysterious 13th century tower is located and believed to be connected to the city via those tunnels).