Could i buy 2 spainish sims to use in unlocked phones for a two week holiday to keep in contact with my teenage children. Taking advantage of the free time usuage between mobiles?

can i purchase 2 x £19.99 spanish sims which include £5 credit to instantly use the free allowances in contacting each other on a two week holiday in spain?
One for parents one for teenagers.... Bit like a walkie talkie
To send texts and short calls each day , within the free allowances on british unlocked phones?