As you probably know there are holidays in the Netherlands so some trains are already sold out. However it is still possible to do the journey.
On top of that Eurostar trains have a passholder quota. Book the expensive Eurostar from Rotterdam or Brussels ASAP or take the slower but cheaper route via Lille (feel free to ask for advice).
Look one train at a time so you see all options.
Afternoon departures are available from Rotterdam or on Monday early morning too. A few more from Brussels which you can reach with the hourly Intercity-Direct.
Via Lille it would look as follows :
- ICD Rotterdam Centraal - Antwerpen-Centraal 08:11 - 09:16
- IC Antwerpen-Centraal - Kortrijk - Lille-Flandres 09:36 - 11:51
- quick walk across the square or you leave Rotterdam an hour earlier and have a little break :)
- TGV Lille-Europe - Paris-Nord 12:05 - 13:24 10€
- RER D between stations (Interrail not valid)
- TGV Paris Gare de Lyon - Barcelona-Sants 14:42 - 21:27 35€
Maybe also possible to avoid Paris, boarding a TGV in Brussels (passholder quota) or in Lille.
Third option via Marne-la-Vallée (Disneyland) :
- ICD Rotterdam Centraal - Bruxelles-Midi 07:11 - 09:16
- TGV Bruxelles-Midi - Marne-la-Vallée 10:17 - 12:08 20€
- RER A to Gare de Lyon (Interrail not valid, 5€ ticket)
As you can see this route towards Spain is very popular and especially does not have many trains...