I want to rent a car for a couple of days when in the Netherlands. Would it be better to arrange it online before I go ... or just go to Schiphol and pick one up?

Definitely better -- cheaper -- to arrange the rental ahead of time. But the Netherlands is densely covered by public transportation. Are you sure you need a car?

Arrange it before you get here. Look at the prices for different pick up points too. It may be cheaper to collect it away from Schiphol.

A car can be very useful in the Netherlands if you want to explore more than the usual tourist trail. If you intend to stick to that then public transport is as easy and possibly cheaper.

We'll mostly get around on public transport but also want to see Kinderdijk and Kroller-Muller museum. Seemed like a car might be easier for these.

I'd say there are very few places not reachable by public transportation. for the Kroller Muller museum a car might be easier, but it depend on where you are coming from. for example, if you are staying in Amsterdam you might get stuck in traffic trying to get in or out of the city.