Have you tried Sawyer's Bowladrome in Northborough? It's candlepin bowling, a small place with about 4 or 6 alleys. Kids can handle the small balls. There's a fee for bowling (per game) but you can use a couple of alleys so the kids don't have to wait too long - usually 3 or 4 kids per alley, and a grown up to keep score (unless you don't bother with that). There's a small fee for shoe rental but I think they can bowl in just socks too. There's a free party room and you can decorate, bring in your own cake and goodies, etc. There's a pizza place across the street that works well. A lot of people like it because it's different from all the other play spaces. We have found them to be extremely helpful. It's just off the main drag where Route 20 meets Route 135, behind the building that used to be the town hall and now has a restaurant and (I think) a health club). There's enough free parking in the small lot. We had a great time there for many years. It's a good place for a dozen kids or less. Here's the link:
http://northborough.patch.com/listings/sawyers-bowladrome