Hi CsG29,
You mentioned that last year in late April was your first camping experience. Be prepared for VERY different weather this year. Last year Spring was about a month early, so the conditions you experienced then were more like what we normally see in May. Much warmer and drier than seasonal norms.
This year is a more typical year, with ice-out (ie the ice melts off the lakes) forecast for late-April or early May. You can take a look at Algonquin Park's ice-out report for more details:
http://www.algonquinpark.on.ca/news/ice-out.php
That said, if you do go camping, I suggest you do not go canoeing. Even if the ice is gone, the water will be "cold enough to kill you" as we say. I know you won't plan on going in the water but nobody ever does.
I don't think you'll miss canoeing, though! There are so many great hiking trails in Algonquin Park that you could spend days sampling them. You can also rent bicycles to try the bike trails. The trails will definitely be muddy. Bring water-resistant boots, extra socks and a spare pair or two of shoes. Dress in layers and have a waterproof top layer in case of rain. Sometimes we get gorgeous hot sunny days in April, 25°C or higher, and it feels like summer (but no bugs!). At night, though, it can drop 20° down to single digit temps.
A heated yurt would be your best bet for camping. Even then, you'll want a three-season sleeping bag. Consider also some alternative accommodations. I've heard good things about Wolf Den Hostel near the west gate of the park. I believe they have some dorm-style accommodations which might be just as cost-effective as camping.
http://www.wolfdenbunkhouse.com/
Or even stay at a hotel, motel, B&B or cottage resort nearby and drive into the park and/or check out the surrounding area. It might not be the same as waking up in a tent, but you can still spend your days out in the woods and maybe even find a place with a fireplace or outdoor fire pit to get some of that camp atmosphere in the evening. If you go this route, see if you can find a suitable place that's participating in the Fuel & Fun promo, which includes free $50 gas card and $50 vouchers to use for dinner, bike rentals, etc.
http://explorersedge.ca/partners/
As for Killarney Park, I think they'll be experiencing similar weather patterns as Algonquin Park. Plant to hike and/or bike but probably not go canoeing. You can hike to the Crack from the George Lake campground and it's a great full-day hike. There's a short cut you can take from the road but if you've got the whole day in front of you, go for the whole shebang.