Hi,
Very nice!
Two questions:

• Will the user be able to change the level difficulty by pressing the Maximum difficulty selected entry button? This would be a nice and easy way to change it if needed?
• Will the user be able to choose a dataset that is has a difficulty level higher than the maximum difficulty level? Exemple in this case will the user be able to clieck on Table de 6?

Great job

@echarruau :
-This "maximun difficulty selected" entry was initially just meant as an info of what is currently selected in the list. Maybe we could also use it as you suggest to auto-select a range of activities, but this should be better handled by the global difficulty filter (the one we already have).
-The only thing that should prevent the user to select a certain difficulty of dataset should be the global difficulty filter (in which case we must display a message to indicate those are disabled by the active filter).

The whole interaction with the global filter is something I forgot to add in the mockup, I already discussed it with Johnny since then and have a good vision for it, just needs a bit of time to make the mockups (hopefuly next week).

"-The only thing that should prevent the user to select a certain difficulty of dataset should be the global difficulty filter (in which case we must display a message to indicate those are disabled by the active filter)." -> or simply "grey" them and put them inactive so that people still know that more difficult objective can be achieved if you change the difficulty level?

Yes sure, greying them in the list as "inactive" is what I meant by disable them. Just needs to add a reminder there that those are disabled by the global filter.

I think the possibility to have non exclusif choice in multipledataset config menu will cause problems in smallnumbers where we want to follow a precise plan to teach/train numbers.

I would really prefer to stick to "cumulative dataset" principle rather than "exclusive dataset".
The reason is to be able to have all datasets selected by default, to not force users to select datasets.

Datasets selection should be an option reserved for "controlled" use like in school or under parents control ; for other case, it's still better to have all levels activated to allow user to proceed to next levels without the need to select another dataset.

PS: see this task with the general TODO for multi-dataset to see how it fits the whole design idea: https://phabricator.kde.org/T12632