This is the easiest way, but:
[1] Bike capacity on Dutch IC-trains is limited. If you get the crappiest of all Dutch trains (for train nerds: model ICMm-IV), then you'll barely fit 3 bikes into one carriage. You may have to split your family, temporarily, into duos.
For those not resident. If you download the OV Info app, it will tell you what type of stock each service will use, as well as give live arrival/departure info.
With that information, for Dutch IC trains the bike settings are:
- VIRM 4 - space for 2 bikes at each end of the train, I think it' the 2nd set of doors from the ends.
- VIRM 6 - Same as the VIRM 4, but an additional space in the middle for 2 bikes. Note that while officially it's max 2 bikes per space on the virms. I've seen 8+
- ICM 4 - These have space for 2 bikes, at each end of the train. Enter via the end most set of doors
- ICM 3 - Space for 2 bikes at one end only, entry via an end set of door, Which end arrives at which position of the platform is a roulette. ICM 3 is often run in a dual config, with two of them. Sods law says the bike spaces are at the opposite ends, if one is full, it's a sprint to the other end in hope it isn't.
[2] Buying a bike ticket for the stop train to Dusseldorf is a pain in the ass. It's possible via the DB navigator app (not at Arnhem Centraal afaik). The obvious buttons to click are: Transport Associations, Rhein-Ruhr, Select Ticket, and then whatever you need. At least, there now appears to be an explicit Fahrrad-ticket, in the past it was one of those mysterious Zusatz-tickets.
Oh, you need to pay for a bike ticket on that? oops...
J