Hi there and welcome to Family Vacation Critic. I'm not the editor but I can provide you with a little more information about Niagara Falls as I live about an hour from the Falls on the Canadian side and have been many times with my family.
Typically, June is a lovely time to visit. As the above poster noted the weather has been unseasonably cold this year but hopefully it will start warming up quickly!
Later in the summer the weather tends to be quite hot and humid and the Falls area gets very busy after summer vacation starts. Most Ontario schools break for the summer sometime between the middle and the end of June.
Niagara Falls has all kinds of activities that would appeal to your family. The Maid of the Mist is probably the most popular activity - you don a blue plastic cape and head toward the falls into the mist and you can get quite wet. The Journey Behind the Falls is also increasingly popular. There is a Butterfly Conservatory just outside the downtown area, an IMAX theatre and an observation deck at the Skylon Tower which are all popular. Marineland (similar to Sea World) is also worth spending a day.
Clifton Hill is an area of the Falls that pre-teens and teens love. There you will find rides, the Niagara Skywheel, Ripley's Museum, and glow in the dark golf.
If you are spending several days in the Niagara area then I would definitely recommend a trip to the charming town of Niagara-on-the-Lake which is about half an hour from Niagara Falls. There are lovely boutique stores to visit, horse and carriage rides and it is the home of the Shaw Festival. If your daughters are interested in theatre then they might like to see My Fair Lady which is playing there this year. We plan to attend with our daughters this summer (they are 14 and 7).
The Niagara region is also well-known for its wineries and there are several that offer wine-tasting tours if that would be of interest to you.
Niagara Falls has many hotels that appeal to families. One of the most popular is the Great Wolf Lodge resort and water park. There are other hotels that have indoor water parks as well.
There is definitely enough to keep you busy in Niagara Falls for several days. I would recommend having a look at the website www.niagarafallstourism.com
for more ideas.
If you have any more questions we would be happy to try and answer them. Have a wonderful trip!