Best countries for traveling with little kids?

Pretty well any of them based on your interests , kids absorb what you give them . Never experienced any culture in Europa that doesn’t like children .
English is fine - most places you’ll
Get along ok without much of native language as long as you know please and thank you .

That said , my kids always like some outdoorsy stuff , beaches, lakes , or mountains.

Portugal, people are so kind and helpful.

Everywhere!

I've found Italy and Spain to be very child focused, it's difficult to go anywhere where your child wil not be fussed over however if you aren't comfortable with strangers giving your child an innocent hug and kiss then you may not like thees two countries.

As a European, with children that age and at that time of year I would be looking at a beach/pool holiday but I accept that people don't often cross the Atlantic to spend time around a pool.

Switzerland was great for families. They ride for free on the train and there are lots of discounts. Plus the double decker trains have little mini playgrounds on them on the 2nd floor. Everybody in Switzerland seems to speak English, even though German/French are the official languages. I'm also noticing a lot of discounts for kids in London as I'm planning a tentative trip there next year.

Also, if you're open to the idea of a cruise, a Disney Mediterranean cruise is absolutely perfect with kids. We did one last year that started in Barcelona and did Cannes/Monaco, Pisa, Rome, and back to Barcelona.

Germany and Denmark.

Lots of family attractions, zoos and nearly all paces are very kid friendly. Most Scandinavians speak English because most US movies are delivered with subtitles only. Good to know: the Danish have no word for "please". Instead they have other words you can only feel but not translate such as "hygge" (fun facts).

In larger German cities such as Berlin English is not an issue.
Multi-cultural Berlin has also a lot of kid-specific special events, e.g. famous Berlin flashlight open-air concert (always in September in amphitheatre like Waldbühne). Or very much fun is every year the kid's carnival of cultures in May / June. So much to do and explore, also beyond Berlin. European must-have with kids is Hamburg's Miniatur Wunderland.

My kids are 12 and 9 now and I remember the little kid stage well. Mine were (still are!) kind of active and not quiet. They are well behaved but when little weren’t great with quiet, formal situations.
Give them a square where they were free to run around and they’re happy. Make them sit for a 5 course formal meal...not happy. They can do it now but still we prefer more laid-back places.

The Mediterranean countries are GREAT. People are friendly, kids are welcome everywhere and they can behave like well-behaved KIDS not mini-adults. Italy and Spain are perfect. As is Greece. Also the food is excellent and familiar.
It's great also in that you can easily get a mix - beaches or other nature areas, museums, ruins. Don't just go to "kid specific" things - find ways to make regular things interesting to kids. For example, ruins. Ruins and Greek/Roman gods will be fascinating to lots of kids if you present it in a way they will like.

Greece uses A LOT of English because very few tourists learn Greek. Italy has plenty of English but learning polite words in Italian is a good idea. Spain’s cities are the same but smaller towns especially in the south will have less English. People are great at working with you but learning a bit of Spanish basics would be very helpful.

Where we DIDN'T/WOULDN'T like with little kids is Paris, Austria, and Germany. Too much formality and expectations and quiet. We recently went to Austria. Had a great time but had to keep telling my younger one to use a quieter voice. He’s a loud talker, not yelling or being troublesome, just talking. A lot. The expectations for him to be quiet was stifling. Got to Italy and suddenly he didn’t seem so loud, he was talking normally for Italians :) which meant we all could relax and not tell the poor kid to quiet down every 2 seconds. He learned quiet voice in time, so it was fine but for younger kids who might cry or just be fussy toddlers it would be really tough.

I lived overseas for 9 years between 1981 and 1991. We traveled with kids everywhere. Kids ranged from age 5 up to teens and young adult.

All four of my kids (including step-kids) love to travel. Note, my Son was the youngest and he doesn't remember much of the travels from age 10 and earlier.

Here are some tips:
1) Museums will eventually bore young children. They might be awed for 15 minutes, but beyond that forget it.
2) I still believe that you expose kids, especially over age 10 to important historical and cultural sites. My kids all loved the Sistine Chapel, even when my Son was 7.
3) Kids loved things like the gondola ride in Venice, Coliseum in Rome, Eiffel Tower, going up on the glacier at Yungfrau in Switzerland.

4) Don't worry about the language.
5) Try to incorporate things that your individual kids would like. My daughter was a teen and loved to ride horses. Also, she loved history and I gave he a couple of books to read about places we were visiting. That helped her to have a background in the history of where we were visiting.

My kids loved, and were loved in, Italy.

A favorite photo is of our then three year old daughter being fed by our waiter in a Siena trattoria.

Paris was perfect for them at 8 & 10 last year. Old enough to keep up and appreciate what they were seeing and young enough to be enthralled.