If it’s a family holiday I’d imagine you’ll be in close proximity most of the time.What does that achieve, besides arguements. Its the data that costs, not the 1 euro SIM.
Pop one sim in your phone, pay for one months unlimited data. Everyone sorted.
tbh the whole issue is a non story imo. Wifi is eveywgere these days, and if you’re somewhere it isn’t. Enjoy yourself whilst there instead of being on your phone.