My kids just love the idea of being with grandma and grandpa. They don’t have to do a thing! 1 and 3 are ages that don’t need entertainment (speaking from someone with a 5 and 12 year old who do need it!). What about just going for a walk, or playing ball in your yard? Blowing bubbles? I don’t know if they are boys or girls but my daughter loved to have tea parties with grandma when she was 3. They did (still do) crafts together. Lots of coloring! Grandmas just seem to have undivided attention and kids sense that. And my kids loved that grandma has a drawer with all their stuff in it - even though almost every drawer in our house is dedicated to their stuff! Enjoy your time with them!