Hi. I am planning to visit Europe in the month of May. It would be three of us, including our 4 year old kid. We are planning to arrive at Amsterdam and exit from either Milan/Zurich or Munich.
The rough planning for almost 3/4 of the trip is done. Now we have approx 3-4 days pending. We have planned our visit till Switzerland (including Interlaken and Lucerne). Now we are bit of confused whether to proceed to South of Germany(Neuschwanstein Castle, Black Forest, Munich etc) or Austria(innsburg,hallstat, Salzburg etc). Could some some share us some valuable suggestions?

I'm a bit confused by your route, but I'm guessing that you plan is one of
1) Netherlands, Belgium, France, Switzerland then where to go next?

2) Or have you done Netherlands, Germany, Switzerland then where to go next?

If I'd got to Switzerland I'd just keep going and visit Italy.

I note you are only in Europe for 20 days so you are traveling fast to cover all those interesting areas. Which fortresses have you visited, which castles, which swimming pools, etc etc. Perhaps you need to spread out how fast you consume say the Mosel or the Rhine.

I suggest you layout your plan for us by night so we can propose to you how to make the best of it.

Last fall, we spent time in Switzerland, Germany and Austria with our "kid" but she's now 30-something so travel was a lot easier for us. And, she was great at using her smart phone to find great places to eat. We saw a lot in only 8 days.

Here is our trip report: 3 Countries, 8 Places, 8 Nights, Awesome!

Following those 8 nights with our daughter, we went to beautiful Lake Orta in Italy with a day trip to Lake Maggiore. The previous fall, we spent three nights on Lake Como and an equal amount of time on Lake Garda, both great spots for families in northern Italy. You can see trip reports on tase in our personal link.

Train or car?

A somewhat serious visit of the Netherlands, the Westernmost part of Germany and Switzerland isn't possible in 2 weeks anyway.
But without knowing your itinerary and your interests, it's impossible to make a proposal.

Well they have planned it out except 3-4 days and that's what they're asking - hope they figured travel time between bases - relocating takes time.

But some folks like to travel fast!

Hi,
Interlaken is not a destination in itself but rather a spring board for the amazing region of the Bernese Oberland. I have travelled a few times with my children up into the car free villages of Muerren and Wengen. You are almost there just 50 minutes from Interlaken. It would be a shame for you to stop just before the good stuff. There are children friendly trails and even playgrounds for your 4 years old. Think about it: you would be IN the mountain not looking at them from a distance.

In terms of what to do next Germany (Bavaria) or Austria( the salzkammergut) either area are great, and really it depends of what you want to see. It would help to see your overall itinerary.

Again, Wengen is great for kids that age folks here say - and as toujoursvoyager says easy to get to by train. May may not be the greatest weather though and it's off-season with some key lifts down for repair.