YES! Your kids can hang out at the library - its a wonderful library and people are nice and helpful. The downtown section is easy to walk about. The Goodwill Park is in the center of town and it is a great place for kids of all ages. There are also 4 casual restaurants in the downtown section that welcome kids - they can get snacks, sandwiches and ice cream.
During school starts and endings there are crossing guards at key places to offer more safety for kids to cross the street - therefore making it possible for the kids to walk to town after school.
Many of the shopkeepers will get to know your kids - which makes our town feel so comfortable.
I have lived in Holliston for 18 years - I love the town - my family has enjoyed this town. I raised 2 sons here and they have turned out more than OK :)
So come on by for a visit - I will personally give you a tour - it would be my pleasure!