Hi Irina, welcome to MobileRead.
I would suggest you a Pocketbook 602 or a Boox x60 or an Ectaco Jetbook Mini. They are amongst the readers who read the widest range of different formats including pdf, doc and fb2 files. fb2 is a file format that is very common in Russia.
Pocketbook delivers their readers with English and Russian dictionaries, for the Boox you will find free stardict dictionaries on the web.
Reading pdf files on a 6" reader might be difficult if the pdf files are edited for A4 pages. Pocketbook and Boox handle pdfs quite well but it's not perfect.
Why did I suggest the Jetbook? It doesn't cost a fortune so the loss won't be too gross if anything goes wrong. Children are children!
But the screen of all readers consists of glass and if not treated with normal care they might break.
Maybe you want to take a look at this site:
http://www.dulinsbooks.com/
It's owner dulin's books (
https://www.mobileread.com/forums/member.php?u=483139) is a member here too and very helpful.