How far out of the city do you want to stay - in travel minutes? Do you want to stay in one place all the time before heading for de Efteling?

I think about 20-30 minutes out of the city. (I have no idea if this exists, I an just trying to find if it is possible). And we do want to stay in one place before heading to the de Efteling area.
I see that stays are usually for a week, midweek or weekend, so 1 night here,2 night there does not seem so do-able.

If you are looking for a holiday park then there is one in Zandvoort, on the coast which may suit. Easy half hour train ride into Amsterdam. http://www.sunparks.be/EN/BE/parks/zandvoort_aan_zee
Otherwise a little but further out, and maybe better for the car trips part of the stay there is one in Loosdrecht, near Hilversum (from where you can get a train - or drive to Weesp and get the train there) or the Eemhof in Flevoland.

http://www.village.nl/engels-verhuur/introduction-en/

http://www.centerparcs.com/EN/SK/villages/eemhof

There are also several in and around Noordwijk/Katwijk, but they are further away from Amsterdam.

I don't have experience of these. I know the Eemhof is good but can't comment on the others.

I doubt you will find a farm which will offer the accommodation you are seeking.
How long are you wanting to stay?

I just found these too, which are very close to me, in Baarn, which is about 40 minutes by train from Amsterdam, and situated in the centre of the country. These houses are set in lovely woodland, near where I walk my dogs!
http://www.belvilla.co.uk/search/#land=nl&rgo=utr&apers=4&output=json

I can have a look further if you want me to. Sorry I can't offer any personal recommendations.
Do you want two houses to share one?

Whilst down at de Efteling consider also visiting Beekse Bergen safari park, and Den Boshc is also worth a visit. The National Park Loonse en Drunense Duinen is also worth a visit.
Sadly their website is only in DUtch but it gives you an idea:
http://www.nationaalpark.nl/ldd/default.xml

If you opt for the Baarn park then you can also visit the Soesterduinen - a big sand dune area in Soest, and also the Military aircraft museum (free!) in Soesterberg.
Paleis Soestdijk (home of the late Queen Juliana) is also in Baarn, and whilst a guided tour of the palace is not much, and I think only in Dutch, you may enjoy the gardens. For the guided tours you have to book online, for the gardens you can just turn up and pay at the gate.