Hi, your minis will be fine without grass as long as they get all the fibre and nutrients they need :)
We have a 0.6 acre paddock and 1 15hh horse, he has loads of room to gallop about in and in winter grass levels are low but we always can spend all the money we have on hay!
Really love you mini btw, so cute and im so jelous, im going to have one one day
My fat horse in a our field, he has eaten it all now :P but he is now lost the weight