Well if it's a nice day and you don't mind zoos, the zoo in Wpg is cheap compared to other places.
The CHildren's Museum is good once they're at least 18 months, more like 2 years, and walking well. I feel like we get our money's worth although it's not that cheap.
The Manitoba Museum can also be entertaining for tots in many ways with the stuffed animals and dioramas and the 1900s city replica area, and the big boat!
The pools - Rady JCC has a salt water pool and so does St. James Civic Centre. Pan Am Pool has a nice heated kiddie pool area.
Aside from that there are various playgroups at community centres throughout the city. Some I know of include R A STeen in Wolseley on Wed mornings - free and Earl Grey in Crescentwood on Tues and thurs mornings (former is free, latter is $3/kid).
Hope this helps!