Hi
They can have one tablet in a days apart from the diet after breakfast . Choose the multivitamin which is natural or if you go for synthetic multivitamin then check few things like
Vitamin b6 should be pyridoxal 5' phosphate(PLP) instead of pyridoxine HCI which is not active form of vitamin B6 .
Same goes with
vitamin B12 methylcobalamin instead of cyanocobalamin.
Taking about minerals they are poorly absorbed but when bound to an amino acid which is termed as a Chelate the are absorbed much better.
You can check these things before you buy a multivitamin for your parents or your self .