Slovan had been fined and handed the sanction by Uefa for previous racist behaviour from supporters.
However, the European football governing body's rules allow children under 14 to attend behind-closed-doors matches free of charge.
It was thought up to 15,000 (including an adult for every 10) were allowed, but it turns out there will be more than 21,000 from all over the country at Tehelne pole – and it will be Slovan's biggest attendance of the season.
The club confirmed on Twitter the entire ground will be filled.
Only 200 Wolves fans were offered tickets, which were snapped up swiftly at £55 each – the price set by the home club.
Slovan were fined for racist behaviour and handed another fine ‘for the blocking of stairways, throwing of objects, invasion of the field of play and insufficient organisation’ in a Europa qualifying game against PAOK in August.
Those two fines came to £82,000.
Slovan were also ordered to display a banner with the wording ‘#EqualGame’, with the Uefa logo on it, against Wolves.
Their last Europa home game – a 4-2 win against Turkish team Besiktas – was watched by 5,000 children despite a behind-closed-doors sanction.
Wolves have three points, Slovan four, going into the Group K clash on Thursday (5.55pm BST).