any character can be studied of course, but especially considering Geobra being a german company, a Hitler clicky would be too controversial. Vlad the Impaler might also be a bit too gruesome.
These little comics that come with the clicky would just give basic information, pretty much to let the children know who the person was and what they where known for. It wouldnt be a deep biography since that would be boring to most children. The story of the person could be told in maybe ten pictures with a bit of text.
Mostly the intention of this history series would be to encourage children to ask their parents or teachers about these persons, and the older children could be inspired to look it up themselves. Just spark their interest a bit.
Whats controversial can definatly be a matter of opinion and where you live, that is why its important that these little stories stay with the facts and treat the historic figure with respect. Teach the children a bit about their mistakes as well. Napoleon did bad things, but also did some good things for France. Children could learn a little about both.
Overall, these stories should be kept a bit light hearted just like playmobil. Don't forget that friendly clickies would be portraying it in the pictures. It would pretty much provide the most basic information and a fun fact. (Like Napoleon not being short but of average height.)
Great suggestions by the way, legends such as robin hood could be part of a different series though.
