We are 2 families (with small kids) looking to plan a 2-week roundtrip covering the highlights of Bosnia and Montenegro. Most probably we fly in and out of Dubrovnik.
Any advice on a nice itinerary, which is not too busy, limiting long drives and is kid friendly?
Thank you!

Rather than round trip tickets, which would require you to backtrack, consider open jaw flights. You can search for them by selecting "multi-city" in your search engine. You refer to "drives"-- shall we assume that you plan to rent a car (or two)? In that case, you'd probably want to rent and return the car(s) in the same country, so maybe Dubrovnik and Zagreb? Other airport options would include Tivat or Sarajevo. Please remember that you shouldn't drive for at least 36 hours after crossing multiple time zones.

As for a possible itinerary, up to you! If you haven't already done so, I'd recommend consulting the Rough Guides. They are particularly good for that part of the world.

What time of year? That will make a big difference.

In addition to the comments and questions kja and rialtogirl already mentioned/asked, what are the interests of you, your family, and the other party's family? It is hard to suggest an itinerary as everyone travels for different reasons.

Ages of the children would also be helpful to know. You said "young" which i don't know if you mean toddler and younger, primary school aged, or maybe even a bit older (i.e. 8-10). Kids often have their limits and as such, impacts your itinerary.

Guidebooks and other travel resources is a good place to start. They often will list activities that are suitable for families or even active pursuits (i.e. hiking, water rafting, etc.) if that is of interest.

Hello, we are flying in from Amsterdam, so no issue with time zones. No direct flights to Bosnia and Montenegro unfortunately, hence the suggestion to fly in and out of Dubrovnik. We will indeed be renting cars.

2nd half of August. Can't avoid the high season unfortunately due to school holidays!

Hi, children are 5, 6 and 8 years old. We all like nature (only short hikes though) and old villages/cities. Having a beach day somewhere would probably also interest the kids and maybe some adventure park. We have 2 vegans in our group if that's worth mentioning.

You can find historical places of varying points of history anywhere. So you need to define what history you want to see in the areas you're planning.

What kind of beach day(s)? Lake or sea? For sea, you are limited in Bosnia as there is a very small beach side north of Dubrovnik in Neum. Montenegro has more options seaside.

For lakes, you have even more options in both countries, Ohrid being one but maybe a bit of an outlier depending on your other itinerary points.

Bosnia is quite large in comparison to Montenegro, so concentrate on the South given proximity to your RT airport, and the two regions you mentioned. You didn't mention if you are wanting to spend anytime in Dubrovnik itself, which needs to figure into your plans if you do. I suggest putting all of that time at the end for your departure home, regardless, you should plan on your final night being spent in/around Dubrovnik.

At this point, I think both families need to do some research and talk about what you all want to include, review maps and travel distances (rome2rio is a good start) and keep in mind border crossings are not necessarily in travel distances, so those can add times to yoir transits. Once you have more framework, we can make better suggestions.

I would avoid the seaside in Montenegro in August, It is uncomfortably crowded, expensive and there is unbelievable traffic. Check the mountain areas like Durmitor - it will still be busy, but not nearly as busy. You could stay in Cetinje (the cultural capital of Montenegro, a beautiful small town) and go to Lovćen national park with the Njegoš mausoleum, and also visit Lake Skadar from there. If you want to experience Kotor and the bay a bit without the nightmare of driving there in August, there is a cable car that can take you down from Lovćen. https://kotorcablecar.com/

(there are reports of taxis ripping people off from the cable car to Kotor. Better to have an app like Tesla app or Red taxi app to get around by taxi.)

You should try to self cater as options in restaurants for vegans are fairly slim. Both of these countries are big on meat.

In Bosnia-Herzegovina there are some nice things to do around Mostar such as a visit to the Kravica waterfall (the kids will probably love this) and the town of Mostar is very pretty. Are you considering Sarajevo? If you make it that far, you could add a visit to the Pyramids. I have never been but I want to go. The water there is healing - there is even a famous Serbian tennis player that drinks it
https://piramidasunca.ba/eng/home-en.html?view=featured

If you really want beach time, you might find better options for families in the southern part of Croatia. But as TravelNerd says you really need to narrow down your interests as a group.

Taking your comments into account, I am now considering to drop Montenegro from this holiday (instead visit this country separately outside the high season), and do a combination of Croatia and Bosnia.
We have already been to Split, Dubrovnik, Rovinj, Plitvice and Bol Brac in Croatia, so would like to explore some other family friendly areas. Especially the island of Korcula seems interesting for 3-4 nights (do you need a car on the island?). We would then also like to spend 3-4 days around Mostar and visit some places around there (any tips?). Not sure if it is worth to also go up to Sarajevo with our 5-year old?
Next to Korcula and Mostar (if you agree both are nice), would you recommend another area (mainland Croatia) that would be a good base to spend 4-5 nights?
We would be flying into either Split or Dubrovnik, but again, we're not looking to visit these cities.
Thank you!