Mittensmum , I absolutely empathise with you and I know how exhausting a toddler can be. I don't think it's a big deal to have a short amount of screen time.
I have been looking after my DGC 3 days a week for about 3 years, first my DGD until she started nursery, and now my DGS since 1 yr old, he will soon be 2 yrs. All in all, I've clocked up a lot of hours and some days are up to 10 hrs.
However, I and DH (sometimes) have thoroughly enjoyed caring for them, but indeed it is a labour of love.
In all this time, we've had nothing but appreciation from our DS and DIL, I am sure they realise the value of our help, early starts, late pick ups etc.
I would be very upset if they criticised how I look after
them, they know they are well cared for and they don't have to worry.
Luckily, they both have always taken a good nap after lunch, which allows me to catch up and have a rest.
If you are finding it too tiring, you could always cut down by a day each week, when they can arrange alternative childcare perhaps you will be appreciated a bit more.