Congratulations, how exciting!
I believe it would make sense to start 'younger' than their actual ages. So think maybe 2 and 5 rather than 3 and 6?
At two, many children are only just starting to enjoy stories. Touch and feel books are very popular. Usborne has has large range of those. Also lift-the-flap books.
My own (adopted) DD didn't enjoy books at all for a long time but then grew into them via books based on her TV 'interests' e.g Peppa Pig. Especially those with buttons that make sounds.
'You Choose' is a must, it allows you to model talking about your own preferences and experiences and DC can just point initially, then gradually build up to talking about the 'whys' of their choices. It's fantastic for speech development and also for gaining an insight into their though processes.
DS (not adopted) was a book lover from early on. He enjoyed stories, but also 'non-fiction' so for example books with pictures animal 'houses'.
I'd avoid the Gruffalo for a while. It has this rather insidious message that monsters that are meant not to exist, may actually exist anyway. Quite a few of the Julia Donaldson/Axel Sheffler books have a level of threat. They are lovely books for language development though, due to the superb rhyming. 'A Squash and a Squeeze' should be fine, and 'The Smartest Giant in Town' (though there is a family of mice rendered homeless by a house fire), and a few others.
Judith Kerr's Mog stories, and perhaps The Tiger who came to Tea (though said tiger eats up all the food and drinks all the drink, meaning that the main character Sophie can't at first have supper; might not be appropriate depending on your future DCs' background.)
Nick Butterworth's Percy the park keeper stories are lovely gentle stories featuring animals.
For funny, look at 'Dinosaur that pooped a planet' and similar.