Can you give us a bit more info? What time of year are you talking about? Are there going to be other children in attendance? What ages? How do you plan to entertain 75 to 100 people for this party? Are you looking for some place close to Portsmouth, or are you expecting that a large number of folks will also need lodging? Are you looking for luxury? rustic? and entertaining venue? An outdoorsy thing?
To explore a range of venues, visit the NH Tourism site www.visitnh.gov. CLick on "Planning and Travel Tools", and then on "Reunions." There are quite a few hotels throughout the state with banquet facilities to accommodate you. Yoou might look into renting out the Seacoast Science Center for a private party. Likewise the Montshire Museum, also a hands on science museum, in Norwich, VT across from Hanover. In these and other facilities that may not have their own in-house catering, there are all kinds of excellent local caterers throughout the state. I am sure we or the facility could help you with this.
You might also look into summer camps that might be available for rent in the off season. A couple that I know of close to me are Camp Chenoa, a Girl Scout Camp in Antrim. We rented the dining hall for my 60th birthday party, but they will even rent out the entire camp. Also Windsor Mountain Camp International, in Windsor, NH (near Hillsboro). These and others will have dining and commercial kitchen facilities. They may be able to assist with or help you find a caterer. They also have rustic cabins, if they are willing to have them used and if your guersts are into that sort of camping experience. Plenty of nearby B&Bs, inns and hotels, throughout the state, if needed.
I will echo others. This is not going to be a memorable event or even a meaningful one, for your 1 year old. It is a special day for you, perhaps. I am sure it will be a fun party for everyone. Be realistic about the needs of the guests, especially the kids.