Family in TO so have done a lot of holidays there.
The CN Tower is a touristy thing but a must - if you eat in the (revolving) restaurant then you don't pay to go up the tower. Food is good and they do half portions of anything at half price for kids - tho yours may be a little old for that deal now.
We like Toronto Island - you get a little boat over there, can hire a giant family bike and cycle around. You can swim in the lake but it won't be warm enough in April.
I usually try and get a visit to the Royal Ontario Museum or the Art Gallery (AGO) in, but they aren't free as would be here so you have to commit to the half day.
I like to walk in the ravines - there are lots of footpaths down there and some of them are almost downtown - kind of unusual in a big city.
Finally, the individual areas have quite a lot of character - Queen St East (everything is E or W of Yonge St) is good for quirky shopping, the area known as The Beaches west of the centre has a very alternative vibe. We like to shop and my favourite mall is Yorkdale - on the subway. Prices aren't currently that 'attractive' to us tho...
Not sure about weather - it is continental climate so can go either way at Easter. we were there a couple of years ago in May half term and it was sweltering - so you could be lucky. Good luck you will have a great time!