Glad you have ordered a pulse oximeter it honestly helped me a lot as several times my oxygen levels dropped and I didn't know about it.
I'm recovering from a moderate case I did have a little trip to hospital due to crazy heart rate but some tips for your parents if they develop a moderate case would be the following:
1: Use a oximeter regularly (every few hours in the day) to check levels are not dipping below 95 if staying below 95 on retesting at hour intervals call NHS 24. If drops below 90 then it's an ambulance. Silent hypoxia is a real issue with covid. You don't feel it (I just felt more tired)
2: sitting up will help their lungs despite wanting to feel like laying down I used a beanbag to keep me more upright including for sleeping
3: if they develop burning lung sensation, compression or constriction in the chest feeling call NHS 24 and they will usually get a covid Dr to call them back. A Dr can usually prescribe steroids to help and they certainly do help!
4: Keep fluids up and have them order a online shop which consists of premade soups easy to heat up. Even though they may not feel like eating they need to eat
5: rest as much as possible!
6: pain relief is also key Ibruprofen is safe to use as well as paracetamol
7: don't push it just because one symptom disappears. In total I have had 10 different symptoms.
8: use the zoe app its helpful for listing a load of symptoms and also has very useful info on it.
I hope they only have mild cases but if not they shouldn't hesitate to call NHS 24.
My toddler was also positive and didn't have a single symptom. It's best to test her from 3 days onwards from exposure.