Polly wouldn't be a 'very easy' trip since you have the long 2.6km portage that you'll need to tackle twice (getting in and out of the lake).

Sproule I think is a great option, she'll get to experience portaging but it will be easy and you can double / triple carry if you want to bring some extra luxuries since it's such a short travel day regardless. There are a few decent campsites on Sproule, but it won't feel very remote since you'll have neighbours nearby, you'll have cell service, etc.

Opeongo South Arm is ok as well but I'm not a fan of most campsites in the area and it will be even noisier with the boats constantly passing by. And no portages for her to experience unless you went on a day trip.

If Sunday or Little Rock is available I'd recommend one of those over Sproule probably. There will be fewer people on the lake and the campsite selection will be more predictable. There's only one campsite on Little Rock so you have the lake to yourself, and there are 3 campsites on Sunday but in my opinion, the 3 Sunday sites are all decent to good, while Sproule has a wider range (some are really nice, some aren't) so Sunday has a bit more predictability. And then you can always day trip into Sproule if you wanted.

I'm not crazy about the sites on Polly, personally. One is very exposed so would only be good if the weather is nice, and the other is completely sheltered and wouldn't be a great first Algonquin experience type of campsite. But the lake would offer good privacy since there's only 2 campsites and very little through traffic.

Parkside Bay would be the opposite, I really like most of the campsites on the lake, but the lake will be much busier since there are so many campsites. So it really depends what you're looking for. If it's her first trip and first time sleeping in a tent, I'd prioritize a nice campsite since that's where you'll be spending most of the time. Something spacious with a flat tent spot, pretty views onto the lake, somewhere to sit and enjoy the sunshine, etc. So my vote would go to Parkside Bay because of that.

Thanks so much for the info. Because of the slim pickings and sites booking up rapidly I settled on polly for the first night and then Linda was open so we will move campsites to Linda. Excited for the second night and hoping we get the open campsite on polly. I appreciate everyone's help!