Someone else asked a similar question recently and I think on that thread the consensus was it's a nice idea but probably too fraught with difficulties - see below

www.mumsnet.com/Talk/the_tack_room/3555244-Horses-at-home-offering-livery

It could work really well with the right person but if it doesn't work out it could be a nightmare. And if you are novices it could be difficult to tell one from the other - so many horsey people think they are experts and come across really knowledgeable but turn out to the total bullshitters, peddling 'advice' which is misinformed at best, dangerous at worst! You would definitely need someone with at least 2 as well, as horses shouldn't live alone - assume there's enough land to support 2, allowing rotation and grass to be rested etc?

If you did go for it you would need to be super upfront about what you would expect from them and what you would do in exchange. E.g. do you want formal 'lessons' in stable management or more to tag along with them whilst they look after their horse? How often? Would they be 100% responsible for care of their ponies, including in emergencies, or would you be offering some kind of help/part livery? What about maintenance of the land, poo picking/harrowing, reseeding, weeding, fixing fences, partitioning off/rotating the fields, maintaining shelter and stables - do not underestimate how much work and expense all this takes even for a relatively small bit of land! How often would you want them to let your DDs ride, and who would supervise them, you or the owner? If they are in any way 'teaching' you/DDs they may need insurance for this. What happens if their horse is lame or isn't suitable for a beginner? Have you got any facilities e.g. an arena/school or nice dry, flat area to ride in, or quiet bridleways nearby? If not then you may struggle to find someone.

I think it might be more straightforward to simply rent the land (with all responsibilities for maintenance) to someone on a normal basis and charge fair market rent. Look for someone who seems friendly and is prepared to chat with you about ponies, perhaps ask if your DDs can do some chores for them in exchange for rides more informally? As an owner I'd be happy with this on an ad hoc basis (although my ponies aren't really suitable for novices so I would probably give them £5 for an hours poo picking or whatever rather than offering rides), but I wouldn't want to commit myself formally to an arrangement requiring it on a regular basis, even if I got a rent discount or free rent, as some days I really don't have time to have a child in tow, slowing me down and having to explain what I'm doing, and I have a near pathological fear of being in charge of other people's children around ponies

How much land do you have? You need 1.5 acres for the first horse and 1 acre for every horse thereafter. You also need someone to come take away the muck, fencing suitable for horses, water and, ideally, a stable for dentist/farrier/vet. You also need a secure room for keeping tack as well as a covered area to store hay, straw and feed. Your livery will need access to a toilet. You will need to carry out repairs to fencing and stabling as well as regular maintenance of fields, e.g. poo picking, topping, fertilizing, weed killer, pulling out ragwort. You may find a livery who takes on all these jobs for you but then again they may not do them to your standards. You also need reasonably safe access to hacking - I assume an arena is a step too far, but without one it can be difficult to ride in winter.

One horse is likely to be lonely and become stressed. Two horses may bond to each other and go ballistic when separated, three is ideal.

Finding someone whose pony is suitable for your DDs to ride is a matter of luck. Are you in a really horsey area with few livery spaces? Be careful what kind of agreement you make with the owner. When do they ride, when do you ride, what happens if the pony cannot be ridden for a while, what happens if the pony is unsuitable for your DDs, who supervises the children when handling and riding?

That sounds sensible. Take your DDs to ride at the local riding stables, they will make friends, contacts, find an instructor, etc. and take it from there.