Eurorail pass

Depends. Are you wanting a sleeping compartment? Couchette? Will it be a holiday period? Contact Czech Railways, www.cd.cz/en/.
What itinerary are you planning? Czech and Polish rail tickets are pretty cheap to begin with. Do you want to spend big bucks on a Eurail Pass?

It will be in mid April, and we were going to get a europass for Germany, Belgium, as well as Czeck and Poland. Couchette would be OK unless there's a WC in the sleeper, which I doubt.

I can't speak for the trains between the Czech Republic and Poland, but most, if not all, of the night trains in Germany will have deluxe sleepers with WC and showers (but they are expensive, you must have a 1st class rail pass, and the accommodation supplement is not included with the rail pass). For instance, there is a night train, CNL 457, originating in Oberhausen, stopping at Düsseldorf, Köln, Dortmund, Bielefeld, and others, then on to Berlin at 4:22, Dresden, and Prague at 9:25. From Berlin on, for the sleeper part of the train, you can only get off, not on.

The Prague-Krakow night train is not a German CNL and sleeper prices are much cheaper than in Germany. For approximate prices, see http://euraide.arsdehnel.net/docs/trains/pdf/krakow2.pdf. I think that booking three days ahead is fine if you can be a bit flexible about the type of compartment you'll accept (e.g., I don't know whether or not the doubles tend to sell out faster).

The Eurail Select (4-country) pass does not offer Poland this year; check again next year in case that changes. It is covered by the Eurail Global pass (usually a more expensive option), a Germany-Poland pass, or a European East pass. See also http://www.ricksteves.com/travel-tips/transportation/trains.