I'd just say something like 'we're doing this and you're more than welcome to join us, the tickets are X amount' just be straight about it because you dont want people to presume you're paying and then asking them to and it all being awkward
I completely agree with this.
Both our families love spending time with our son, and love to be involved marking his birthdays in which ever way we choose, just to be involved. So I think it will go down really easy with people and explain you absolutely won't be offended if people don't want to,
The aquarium is definitely age appropriate, it will be a lovely day and our son loved watching everything swimming around and all the different colours and things you don't really see in day to day life.
If financially possible, I would provide all the picnic, but if not, once people have confirmed, maybe say MIL bring dessert, SIL bring salad and so on. So everybody brings a little something for everyone. I suppose with that, only you knew how accommodating and easy going your family are.
I would also broach the subject sooner rather than later, so any anxiety about it is out of the way and you can just concentrate on being excited, for what sounds like, a really lovely day.