It's so kind and loving of you to be so concerned for your granddaughters. Consistency is very important with any kind of incontinence issues, so it's probably best when they stay over with you to continue doing the same routine and practices that they are accustomed to at home.
Using absorbent products on children who wet the bed is a very controversial topic - some people feel that it makes the children feel like babies, while others feel that it helps the children feel in better control of the situation. Either way, it's important to remember that children don't wet the bed to be naughty. It is something their body does for any number of reasons. Quite often it is because the body isn't producing enough of a specific hormone (called vasopressin) at night that is designed to help concentrate urine while you sleep.
Some children also experience bedwetting due to stress, or because they are very heavy sleepers, and it is thought that there is a genetic component as well (meaning that children will be more likely to wet the bed if their parents or grandparents did, too).
Also, keep in mind that most children grow out of bedwetting by age five or six, so it isn't at all unusual that they are still wetting the bed at this age or slightly past this age.
Back to that issue of wearing products - it's my personal opinion that the best course of action is to have an open and honest conversation with the children. Explain that their body is still growing up, and that means that they still wet the bed at night. Explain that you realize this isn't their fault, but you want to help them to be as comfortable as possible. Then ask the kids how they'd like to handle it - getting up at night, an alarm that wakes them up on their own at night, a special bedwetting alarm that might train them to eventually wake up before they have to urinate, wearing an absorbent product, or any combination of these therapies.