Hi, if you look on the BHS website there's plenty of advice. My understanding is that if you need to provide essential care (such as feed and mucking out) then that counts as an necessary/justifiable reason for travel so go ahead, but don't go up there unnecessarily, just to pat your horse etc.
Owners/yard managers are being encouraged to come up with rotas to minimise visits and where possible ensure only one person is there at a time, as your group has done, so sounds like you are doing all the right things. The BHS have a letter you can print off and keep in the car in case of being stopped by the police to explain why you are out (whether or not police are pulling over cars and asking where/why they are out seems to be a very area-by-area thing, nothing in our area but reports of lots of stops in others). So long as you are on your own there should be very minimal risk of picking up or spreading the infection but do follow good hygiene processes and wash your hands etc anyway, and if you come across anyone else e.g. on footpaths, keep 2m away from them at all times.
I would agree, you shouldn't take your DD with you if you can avoid it, make sure you take sensible precautions when you are at the field (particularly if you are feeding/mucking out with loose horses around). I don't mean to teach granny to suck eggs - I have kept away from our yard this week but even last week some of our under-exercised horses were starting to get bolshy and frankly a bit wild, I can only imagine they're worse this week with no riding and minimal handling, so I would be super super careful about being in an enclosed space with them or getting in between a horse and its food, the last thing you want is to end up in hospital having been knocked down or kicked at this time! Even normally very quiet and reliable horses can go a bit funny when there are changes to their routine so I wouldn't trust any of them, if you see what I mean!
You don't mention riding so I assume you are not planning on doing so, but BHS advice is that you can make up your own mind but to exercise caution and basically don't ride unless you have to, again to avoid accidents and taking up NHS resources.