I think you should have a practice first, just with the baby, for say 3 hours. You are completely used to caring for your 3 yr old granddaughter, so know you can happily care for her but the 2 month old is a new addition! See how that goes, what it actually involves and consider if that care can be added to what you usually do with your gd. I think you will be fine, because you are an experienced grandmother who is already caring for a whole day for a lively and lovely gd. Before the day, if you go ahead, have everything planned and prepped ready. Your daughter will obviously be responsible for giving you the baby's bottles & requisites, but you can prepare your granddaughter's meals, snacks, drinks & activities for the day. A 2 month old will (should) spend most of their time sleeping, even if they are a little cranky because they are not with mum, it will pass! I think you will cope with this and be able to look back on the day with a lovely reminder of the love you have for your daughter and grandchildren.