On the Dutch theme, Efteling plan to be reopen by May 20th: https://www.efteling.com/nl/blog/nieuws/20200508-efteling-gaat-weer-open
They will be open before then, with very small crowds (relatives of staff, maybe passholders, etc), and slowly building up so staff can adjust to new routines. The plan come May 20th is that the park will be at most at 1/3 capacity. Their Bosrijk Holiday Village is already open, since people stay in their own self-contained cabins.
Also, a few images that have been circulated by the Dutch Theme Parks and Attractions Association, who have put plans in place for how Dutch parks should go about when opening. These measures have been approved by European safety board TUV:
(Image of Joris en de Draak at Efteling)
Some form pole can be used to check restraints. I've also heard (but not seen a link) that staff are allowed to use feet.
An example of how Python (a 28 seater / 14 row / 7 car train) could be loaded. The idea is that a maximum of 14 guests from a maximum of 5 families can board any given train, with certain rows blocked off.
Other things in place are standard 1.5m distances between groups, encouraging paying by card/apps wherever possible (Netherlands, similarly to Germany, use cash a lot more than we do in the UK), blocking of every other urinal and sink in toilets, etc. Efteling have also been redesigning and rebuilding some of their cattlepen queues.