Welcome to the forums and thank you for doing research!
I agree with everyone else that you will want to keep them indoors. Guinea pigs are really good at hiding illnesses and it's very likely you might not pick up on it if something is wrong in time to get them to a vet, plus all the other dangers people have already listed. Also, one of the main reasons guinea pigs are enjoyable pets to have is because they are so much fun to watch. This is another reason to keep them indoors, preferably your living room or somewhere with a lot of traffic like a study so you can watch and enjoy them. Guinea pigs are not generally as cuddly as other pets like cats and dogs and so while you'd take them out for lap time now and then the majority of the time you'd enjoy them is when you're watching them interact and play in their cage. It's also another reason they need a same sex friend and a large enough space to run and play. Guinea pigs get surprisingly big as they get older and they can run really fast so a large enough cage is very important for their health and emotional well being. You can read all the testimonials on the main menu at the top of the page to see what a difference it makes.
That said, that doesn't mean you can't occasionally take them outside for an hour or two of play time. Just make sure they're supervised, not exposed to extreme temperatures/weather, a pen to keep them from running away, and no pesticides or other products on the grass. You'll also want to check whether there are any potentially poisonous plants in the penned up area. You also mentioned that you have or plan to have rabbits and/or mice. Just in case you were thinking of letting them play together, you should not put any of these animals together. Rabbits can seriously injure mice and guinea pigs, they can also give guinea pigs Pasteurella which is often fatal, and their food requirements are also different. So you will want to make sure to always keep the three species separate (although it's fine if they are in the same room in separate pens where they can't interact).
I also wanted to point out that guinea pigs are quite a lot of work and can be quite expensive. Depending on the age of your kids you will always have to supervise them when they handle the guinea pigs or if they're older to still check on the pigs regularly anyway and take over any slack in their care if the kids are bored or don't care for them properly. Guinea pigs are very fragile and can get injured very easily, guinea pigs should not be pets 'purely' for the kids only to care for. Vet bills can also get very high because guinea pigs need special exotic vets which cost more. Normal small animal cat & dog vets are generally not trained to treat guinea pigs appropriately, even some exotic vets have questionably outdated training so be very thorough about finding a good vet BEFORE you actually need to see one because guinea pigs can go down hill within hours. Worst case scenario you can expect to pay up to $1.5-2k for more serious surgeries and tests so I recommend setting up some savings for the worst case scenario unless you have enough reliable liquid cash to be able to soak up that kind of blow.
Both rabbits and guinea pigs need a lot of vegetables, good quality pellets, and unlimited hay. All this can also add up in costs. For 1 guinea pig you will want to feed 1/8th cup of timothy based pellets per day (unless they're under 6 months old in which case alfafa based), we recommend KMS Hayloft or Oxbow because 99.9% of other brands are junk food or even dangerous. 1 cup of veggies a day, red and green leaf lettuce and green bell peppers should be staples. Plus unlimited quality timothy hay and unlimited water so you will need all this food times two.
Anyway I'm getting carried away
so I would highly recommend reading the following guides thoroughly and fully, these are one of the most well respected and accurate care information guides on the internet that we recommend on these forums regularly and where you can verify the things people are saying:
https://www.guinealynx.info/healthycavy.html
https://www.guinealynx.info/medical_guide.html
And here is a handy veggie chart that tells you how much to feed of what, how often etc. you can also print it out for easy reference. Just make sure to also scroll down to the second post beneath the chart as it has more information and a list of poisonous plants:
https://www.guineapigcages.com/foru...vy-Nutrition-Charts-amp-Poisonous-Plants-List
Some other helpful links and stuff I mentioned above:
https://www.cavyspirit.com/kidsandguineapigs.htm
https://www.guineapigcages.com/rabbits.htm
https://guinea-pigs.livejournal.com/2982433.html
https://www.cavyspirit.com/sexing.htm
(broken link removed)
Then finally, I'd also recommend just reading all the stickies in the different forum categories.
I hope all this helps and if you have any questions at all feel free to ask anything we're happy to help!