For your two oldest ones, you will also be likely looking at school camps soon, so having a decent bag for that will help them have a better time. I'd recommend a mid range synthetic, mostly due to the ease of cleaning. I'd say that a decent bag should see them through highschool at least, so it would be worth spending the coin. On one of the last trips I was on, one of the girls had a brand new sleeping bag, I think outdoor expedition (one of the big box house brands anyway) and the zip was installed wrong! The biggest failing I see is not the bags themselves, but the stuff sacks, either they are too small, so the kids can't get the bag in, or they are made of tissue paper and blow out. (blackwolf bags are notorious for blowing the buckles) I wouldn't be going too high end. I just looked online at the One Planet down-under, looks decent, and you might be able to get a discount depending on retailer and school. But something along those lines, between their current size and likely genetics, you should be able to guess if they need a reg or a short, and even if one outgrows the bag, there should be enough life in it that your 6 year old gets a reasonable handme-down before getting their own. But I'd go with an adult bag for them at this stage, they will start to really sprout shortly, and a bit of kit they grow into is better than something that they know they are growing out of.
For background, I'm in outdoor education so I see a few sleeping bags in a year.
as for packs, look for lots of range of adjustment, there will come a time where you are doing a pack fit every time they put them on. In my experience, most hydration packs won't fit the stuff even a small kid needs for a day walk, and even the small weight can ride oddly, making them not feel good. I'd say at least 10L, so they have room for a snack, jumper, water bottle (don't bother with a bladder, you can't monitor their intake as well) and rain coat, a decently made one should cover them through their adventures up to age 10, and by then those items might be big enough to fill it. A kid can always get use out of a solid day-bag. (pet peeve is kids showing up with a "daypack" that is a string handled gym sack, useless for carrying anything more than the snack wrappers after morning tea)
Some of the parents here might be able to point you at some other specific products, I just go with the generalities as I tend to only have kids for a few days at a time, and someone else is buying their kit.