Hello, GPs are great, lovely little pets. IMO the best small furry pet for children, more interactive than a hamster, calmer than a rabbit. They do need some looking after though (more than a child would do alone) although obviously less than a dog, so you are right to ask! Here's my attempt at answering your questions!
Where is it best to keep them - inside or out? Ours are inside pigs, in summer they go out in a secure run for a couple of hours for 'exercise' (actually they mainly sit under their shelter and eat, they are the laziest creatures known to man!). They can live outside full time in summer and in a heated/insulated garage or shed in winter but I would personally worry about them getting cold particularly in extreme weather, plus foxes are an issue around here.
We have artificial grass in our garden - I'm not sure if that will be an issue? Consensus on the web seems to be that it's fine to put them on artificial grass so long as it's well secured and they don't try to eat it. My pair have eaten all sorts over the years when inadequately supervised (!) including electrical flex cables and a TV remote control with no apparent ill effects, so I think it would be OK. You would probably want to put down some hay for them to nibble on when they are out on it, and accept your grass may take some damage (also you'd have to have a method for cleaning up the poo...)
How much space do they need? Quite a lot, a lot more than Pets at Home or the like will tell you. The RSPCA say 120cm x 60cm x 45cm but bigger if possible. Ours have nearly twice that and I still think that's a minimum really. C&C cages are recommended on here a lot, ours have a two storey version which most people say is no good as guinea pigs aren't great with climbing, but we always feed ours on the top level and the lure of food is a great incentive so now they scuttle up and down all day quite happily!
How much do they cost each month for food, insurance etc? As a rough guess, I would say we spend about £20-25 a month on ours, we buy most things in bulk online which keeps costs down. £5 on fresh veg (they mainly eat peelings and waste fruit and veg from the kitchen), £10 on a big sack of kibble once every three months, £4 on a big bag of hay which lasts about 2-3 weeks and £10 on a big sack of bedding (you can buy cheaper but we use fancy recycled paper stuff which I've found is the best for odour control, see below!). They're not insured, I've found guinea pig insurance to be expensive and only a few places do it, but we set aside maybe £10 a month contingency in case of vets bills, which has been used as one of ours is prone to cystitis.
Do they smell and how often should they be cleaned out? To be honest, yes they do smell a bit and they are also very messy, ours love nothing better than kicking their hay and bedding everywhere. Our cleaning regime is (daily) sweep up of loose hay (they are in a room with hard floors thankfully, although it is a constant battle to stop people treading in it and trailing it around the house) (every other day) spot clean their cage, remove wet patches and poo (weekly) full change of bedding, wash out cage. I have experimented with various bedding types, shavings and straw are cheap but smell and cause them to sneeze a lot because of the dust, plus can hurt their feet, old fleece blanket and newspaper is very cheap but still smells and I found difficult to clean out although many swear by it (poo everywhere when I tried to empty into the bin!), I use recycled paper bedding like this which seems to control the wee smell well (you can occasionally still get a whiff if they haven't been cleaned for a few days).
When I go on holiday is it enough for someone to come in each day and feed them or do they need more? When we go away we have a pet sitter who comes in daily to feed, clean cage every other day and gets them out for a quick cuddle and 'run around' (luckily she is a GP lover so likes spending time with them). Costs us £10 a day. They seem fine with this regime although we only go away for a week at a time, I'd be a bit unsure about whether they were getting enough time out of the cage if it was longer. There are also GP boarding services available in some areas if you look?
Anything else I would need to know? Sensitive issue but although GPs live longer and are less fragile than hamsters, when they do get sick they can go downhill and die fairly abruptly and without much warning. Just something to think about how your kids would cope with that (good life lesson of course). You absolutely shouldn't keep a GP alone so if you do lose one you would have to think about getting another, which can mean an endless cycle of more GPs (not a bad thing 
), but you might want to remember you may be getting into this for longer than the lifetime of your initial pair, if you see what I mean?
That's all I can think of for now, any questions let us know, and if you do get some come back and show pics!