You can find information about the NS Off peak group ticket on https://www.ns.nl/producten/en/meest-gekocht/p/groepsticket-daluren
The Off-Peak Group Ticket is only valid during off-peak hours, i.e. weekdays before 06:30, between 09:00 and 16:00, and after 18:30. At weekends and all day on public holidays. The Off-Peak Group Ticket is not valid on King’s Day (27 April).

The ticket is not valid for travel on the bus from ‘s-Hertogenbosch to Efteling. On these buses you can use your bank card or credit card to pay for your journey. (OVpay) You touch in when you board the bus and touch out when you alight at the Efteling. The journey one way costs €5,85 or €5,94 depending on the bus route.

You could try to install the glimble app. It will give you 10% on the Arriva buses.
https://www.glimble.com/

Alternatively, you could have a look at the combitickets via the NS Spoordeelwinkel: https://www.ns.nl/spoordeelwinkel/uitjes/efteling.html

For €57,50 you'll get a ticket for the Efteling, a round trip ticket (off-peak) from any station in the Netherlands to either Tilburg or Den Bosch and a round trip bus ticket from Tilburg/Den Bosch to the Efteling.

If you haven't gotten the Efteling tickets yet then this method should be cheaper. As normally it would cost per person:

• €22,66 for a round trip off-peak group ticket.
• €11,68 (from Den Bosch) or €6,98 (from Tilburg) round trip bus tickets to the Efteling.
• €42 for 1 Efteling ticket.

€22,66 + €6.98 + €42 = €71,64

I looked at the suggestion of the Spoordeelwinkel. You can only buy those tickets with iDeal, only available with Dutch bank accounts and therefore discarded the idea immediately.

If NS wants to sell these tickets to foreign tourists they really have start accepting other payment methods than just iDeal.

Yesterday I book E-ticket where there is small spelling mistake of a name of one passenger. Is that okay? 

Yes that is okay.