This has blown up into a story
in the Dutch news in the last few days, but it took me a while to track down the real figures as most articles just point to one another without giving a source.
The best I've been able to find is
this link from the KPVV.
Here's a quick English language translation of the graphic showing statistics for all schools for all weather conditions for primary school children:

As you'll see, cycling is still the most popular way to get to school and nationally 37% of all journeys to school by primary aged children are by bike. This increases to 39% in good weather (driving decreases to 27%). Much of the concern is about children travelling independently less often than they used to. You'll see that many adults now accompany their children when cycling or walking to school. However 33% of primary school children still travel independently either walking or cycling. This is not far off what you'd expect when the average age for independence is 8.6 years as 8.6 years is approximately 2/3rds of the way through the primary school age group.
The article explains quite a lot more than just presenting these figures. There are huge variations between schools. For instance:
o The share for cars varies from 6 - 48 % for town schools and 34 - 52 % for country schools.
o The share for bikes varies from 16 - 65 % for town schools and 20 - 44 % for country schools.
o The share for walking varies from 18- 63 % for town schools and 17 - 37 % for country schools.
It's been known for some time that there are these variations. Amsterdam was reported several years ago as having
the lowest first year secondary school cycling rate in the country, for instance. This is perhaps what should be expected for the largest busiest city. On the other hand, it's been
illegal to use a car to take a child to primary school in Groningen for several years so there, the share for cars will be very low indeed.
It should also be noted that there are various special types of school. In this area there are no school buses at all except for those which pick up children who have special needs. There are also far more private cars than average used at that school. I don't know if schools like this they were included in the figures
There is also some analysis of how things have changed. The use of cars has risen between 1994 and 2012 from 26% to 30%. This has resulted in a drop of cycling from more than 40% to 37% and of walking from around 35% to 30%. The public transport / taxi etc. make up the remaining 3%. This is based on figures from SOAB which have apparently used the same procedures over this period of time.
I think the KPVV and others are right to raise this issue and I hope that the decline in cycling and walking can be stopped. Perhaps the alarmist headlines are required in order to wake people up and stop this decline. However the decline is not steep and I suspect there is no other country in the world for which the figures above wouldn't look remarkably good.