Due to coronavirus protocols I am now considering whether I am able to digitise the picturebooks I want to use for research purposes with children. Maybe I would have to approach individual author's/author's estates?
That does not sound like it falls under the 'fair use' guidelines, so I suggest that you contact the publisher(s)/estates for permission or contact your university counsel for advice. If you are making only one copy & it will be on a secure server, the estates are likely to grant permission.
I don't know much about this, but hope that your research with children goes well. I don't know how we would have managed had the COVID-19 pandemic come before there was the Internet! I do hope you are able to get the necessary permissions to convert the picture book into an ebook.
I think you'd have to contact the publisher and author for this, as it wouldn't be fair, but if you're going to put indications that the material will only be used for research purposes and not for anything else, then I think it's possible.