Aww, how cute :3
It's a great idea.
They are precious!
Entertaining kids during the summer can be tough. Some things that I've done:
1. Painting--both finger painting and water color. Messy, but it keeps them occupied and the results were pretty interesting. They always were and still are proud when I display their art.
2. Tie dye--just as messy as above, but kids really get into it! The supplies are cheap and in the end they will have a colorful shirt or socks which (if they are like my girls) they will wear until they fall apart.
3. Walking. Whether you live in the city or suburbs/country, this is good if you take them to an interesting area that they have never seen before. Some hits with me were Chinatown in the city (there were so many interesting and different things to look at and eat) and to a swamp which hosts a lot of wild birdlife. Bring binoculars for the nature walk and give each of the kids one of those cheap disposable cameras so that they can take their own pictures. Then they can make a scrapbook.
4. I forget how old your girls are, but I would give them boxes of my old clothes, scarves, hats and jewelry. They played dress-up but also would make a play which they presented to us. With photos!
If you need any more suggestions, ask--I have the experience!