Yes, you can buy an e-ticket Samenreiskorting

https://www.ns.nl/producten/en/meest-gekocht/p/samenreisticket_enkele_reis_daluren (single trip)

https://www.ns.nl/producten/en/meest-gekocht/p/samenreisticket_retour_daluren  (return trip)

Please note, with this e-tickets you are not allowed to be in the train during peak hours, while you with a seaseon ticket on an ov-chipcard can check in during off-peak and can continue your trip during peak hours. 

Note: this only applies if your parents are travelling with you. If they're travelling alone they are not entitled to any discount.

I read on the website that is the journey is extending to peak hours, then I should buy a combined travel ticket and this will be only available at the NS ticket machines. 

Can you confirm that with combined travel discount, I can travel (part of my journey) in peak hours? I will then figure out how to get these at ticket machine?

That is only possible if you load (“purchase") the combined travel discount on your OV-chip card. So your parents would need their own OV-chip cards.