[xyzzy123]

Keep them out of easy reach but don't go to crazy lengths. If you can, talk to your 5 year old. Ideally, involve them in the process of making sure grandma takes the medicine. Kids can be very responsible when they understand something is important, and why.

Of course every kid/situation is different and needs to be met where things are at (and not where you wish it would be), but I think it's worth trying if you have bandwidth. They may surprise you.

When you think about it, most 5 year olds have ready access to very dangerous things; they can turn on the oven, toast things (not necessarily food), they often have access to sharp knives with a minimum of effort, etc. What keeps them safe is their understanding of the risks of those things. The real problem with grandma's pills is that the risk of them is not "accessible" to a child in the way those other things are.