I cannot say for every country since the rules are still different for each country, but I can tell about my own country. If coming to the Netherlands, you won't need to test to enter the country if you're coming from a safe country. It's best to check regulations shortly before departure. It is likely that many countries will be “green” by the time you're travelling. This means you can travel freely between these countries. Keep an eye on this website: https://reopen.europa.eu/en 

Thanks for informations Mattheas. I have seen the website (https://reopen.europa.eu/en) and I think it has useful informations but on these site I haven’t seen the test places for tourists, especially for children nor prices. That’s why I tried to ask on this platform. I was hoping that someone had personally tried one of these test places and could help me.

Usually there are next to the Airports/Major Stations and even tourist sights test centers For Germany the price for a Fast Antigentest costs from 20-40€ depending from the Testcenter operating Company PCR Tests starts from 50€ with up to 180€ for a quick PCR test. Few countries started to offer free tests if needed to get back. Best in my Opinion is to check step by step Country / City and where is everything.

If you may have a Route in mind i guess we all can try to collect the informations that you need.

For my august trip it took 3 hours to check the actual rules and restrictions but every day something change. Germany - Italy - Slovenia - Austria - Czech Republic - Croatia - Austria - Netherlands - Germany. My Route make sometimes no sense but as iam trying to catch new trains like Trieste - Ljubljana - Vienna, Prague - Split/Rijeka, Split - Vienna/Bratislava and Vienna - Amsterdam such strange Route is the result :)