I have to admit my immediate and instinctive reaction was absolutely do not do this, but I suppose in your particular circumstances and with a lot of caveats it could work out. Some of my 'ifs' /questions I'd want satisfactory answers to would be:
-Is she definitely, actually fully sound in walk and trot, including on a hard surface and on rutted/uneven ground as you would encounter out hacking (not just on a soft arena footing)? With the greatest respect to riding schools who do a hard job, even the most reputable school's definition of 'sound for light work' and yours/mine are not always aligned! And an unsound pony is never going to be a completely safe and reliable pony IMO.
-Does she hack at the moment (+/- lead rein), or is she used to arena work / lessons only? Change of routine and workload is a big thing for all riding school ponies going to private homes and they can be quite unsettled and sometimes challenging to manage, even if in theory their new routine and workload is better suited for them. The last thing you want with a nervous child is a tricky/ anxious pony!
-What are her management needs, does she need specialist shoeing/trimming, grazing /weight management, special diet and are you able to maintain that ?
-Are the school looking to charge you anything for this loan and what is the contingency plan if it doesn't work out for you? I'd assume if they aren't charging anything it's because she effectively isn't up to a school pony workload anymore (schools aren't charities after all and usually aren't in the business of giving useful working animals away for free), and therefore if it doesn't work out with you or she doesn't stay sound in work are they realistically going to PTS? Which isn't a terrible outcome for the pony or anything, but not sure i'd knowingly be risking putting a sensitive young child through that kind of emotional wringer? Equally if your DD is likely to increase confidence and want to move on to a more capable pony in the next 12 months, or on the other hand to give up entirely, where does that leave the pony, presumably you wouldn't be in a position to keep her in retirement? Again strictly speaking not your responsibility as loaner but again in the real world we get attached to the animals in our care and wouldn't want to be the indirect cause of an elderly/disabled pony being passed around...