Using Mifi and data sim in France

HI, I hope you can help me. 

We are travelling to Brittany soon, our first time caravanning in France. I would like to be able to access the internet whilst there and have 2 children who would also like to message their friends (via tablets/internet not mobile phones). 

I have been looking at using my phone as a hot spot or getting a mifi router and buying a data only sim. 

I think that UK data is limited when roaming. 

Can anyone advise on the best way to do this? Buying extra data for my phone is expensive so I wondered whether a mifi router with a data only sim might be best - but does this need to be a french sim in order to prevent the data being limited?

We will probably be using just for messaging, emails etc as I understand watching tv/films is very data hungry!

I have been researching this but it's not my forte really and I have so many other things to sort out before we go I was hoping you lovely people might have some experience and advice to share? 

Thanks in advance!