I grew up about 40 miles from there and although I haven't been for a few years now it has always been fabulous :) 'Innocent and magical' is the right phrase I think. At 5 I think your DS is the perfect age for it too, any younger and the story themes might not make sense (everything is fairytale themed and has a story to it).
If you're going in the Dutch school holidays it will be busy. If you can avoid those, it shouldn't be so bad, especially if you go on weekdays rather than weekends. Here are this year's dates which you should be able to make out even though it's in Dutch, 'Zomervakantie' is the summer holiday. 'Bouwvak' isn't really relevant so ignore that (it's the construction industry's holiday). Holidays are staggered by region so weeks where all three regions' holiday don't overlap will be less busy but still busier than non-holiday weeks. The Efteling is in 'Zuid', but draws visitors from all over. 'Basisonderwijs' is primary, 'voortgezet onderwijs' is secondary.
Never stayed over due to living so close so can't advise on the hotel or food beyond in-park snacks/lunches, which are typical theme park pricing and style - expensive, leaning towards the junk/treat food end of the spectrum, but acceptable quality.
It's not huge, and I think even with a five year old and taking it easy you wouldn't need more than two days (although they didn't have all the shows they have now when I was little, so maybe it'd take longer now!). If you wanted to make a longer break of it, you could also visit Beekse Bergen Safari Park, which is only about 25 minutes away. Again I've never stayed over due to proximity so I've no idea what the accomodation is like, but the safari park is awesome - I love it now even as an adult! There is also a water- and play park next door, although I've never been there so I don't know how good that is.