one month Europe by car: best family luggage ?

It’s been my experience that two smaller bags fit in a car better that one larger bag. That’s especially true for a large backpack like the 60 liter pack.

Soft side stores more easily that hard side.
Packs store more easily than wheelies (rigid parts)

Also - a 60 liter bag is very large. Are you carrying the kids stuff in there too? Are the kids old enough to start carrying their own packs? If the 60 liter is only for the adult then I’d suggest packing less.

In short, there’s other options than the ones you present.

Thanks !

Yes the big bag (or any other luggage) include child stuff. Our boy (10 yeas old) could maybe grab some luggage but not a 40 liters that is full .

My wife and I both carry a 21" Travelpro ultra light carryon bag with swivel wheels. She has an oversize purse and I carry a daypack. We don't check any luggage. And we can go indefinitely with that amount of bags.
We took our two kids with us on one trip, and they carried the same thing. The rent a car we had was a medium size station wagon, and the cargo space held exactly four 21 inch carry on bags.

We are a family of 5 and each has an Osprey 46 Porter. We travel for the same durations you have described (or more). We started when your youngest was 9 two summers ago (she got her own Porter at 10). This would not save you any money this year, but since it sounds like you travel a lot, the value of the packs accrue, in that you can carry them on every time, they hold enough for any length of travel, and all 5 of ours fit in a modest rental car (plus the 5 of us). Personally, I dislike carrying a lot, so 60L is big. I also dislike hard sided cases bc they dont pack down as easily. We love getting off the plane and walking right out of the airport. We figure each kid can take their Porter with them when they leave for college. Lastly, we got them with our REI membership discount ...
I guess the short version of this post is that considering you travel most years, decide on luggage for the longer term.

Your boy can carry a 40 liter pack filled with 20 liters of clothing. He will grow into it.

I’d suggest smaller bags. Even the 46 liter one is too large for several international airline carry on bags.

The RS Appenzell makes a good pack for middle-schoolers. Your 10 year old can handle it. Combine it with some packing cubes to make loading it easier. My daughter has used this bag as her primary luggage for several years now.
Just help him so he doesn't load up on toys, etc.. Younger kids can manage a small rolling "underseat" case. Or, using packing cubes - you can put the youngest child's stuff in your bag. Pack for about 5 days - then, do laundry. Just consider laundry a travel expense like gelato.

We have a few options we use when traveling - my best advice is to NOT bring a ton of bags - we did that last summer (five of us - me, husband, two kids and my mom) and it got annoying quickly. Odds are you can pack both kids into one bag....with access to laundry there really isn't a need to bring much more than 5-7 days of clothes and of that, only a couple are bottoms - tops you bring more, then mix and match for more outfits.

In April it is me and my two kids going for three weeks, looking at my options, the plan is I take my Porter 30 (which is an amazing bag - my husband has the 46, which I find too large for me), both kids (age 7 and 14) get into our split roller carry-on (standard international carry-on size) with one kid on each side, and they each will take their Daylite (which is 13L) and I'll take my mid-sized crossbody bag as our "personal bag" bags.

Worst day scenario, which is how I try to decide on how we'll pack - two sick kids and I have to manage it all - spinner carry-on, topped with porter, then daylite slung over back, other daylite on my back and crossbody worn as usual.....wouldn't be happy doing it, but since I pack as if I have to carry it all in an emergency, it'll work.