No discount for seat reservation/couchette in general, only for tickets.
For Netherlands -
http://www.ns.nl/cs/Satellite/travellers/arrange-buy/ticket-pass-types#railrunner-idBelgium -
http://www.b-rail.be/nat/E/tarifs/specialdiscount/kids/index.phpGermany -
http://www.bahn.de/international/view/en/prices/intercontinental/german_rail_ticket.shtml or group tickets (Lander etc.)
Austria - Family card (
http://www.oebb.at/pv/en/Servicebox/VORTEILScard/VORTEILScard_Familie/index.jsp) or group tickets (Einfach-Raus-Ticket)
Czech Republic - special card need, but better to use passes (like SONE or CD-NET) and group discount
Hungary - best choice probably is 33% discount for member of a family (at least 2 children <18 years old + parents) or even bigger discount for member of a big family (from 3 children + parents)
Slovakia - 50% discount (
http://www.slovakrail.sk/index.php?id=deti-od-6-do-15-rokov)
Poland - 37% discount (
http://www.pkp.pl/node/2153)
All the above discounts are for domestic journeys.
And remember that it is always better to recheck by asking rail companies officials on the spot before buying tickets