OP, you seem like a lovely mum and caring relation to your vulnerable relatives. I don't envy you having to make these choices.
for you.
PP said about children starting school later in some other countries, and that is a valid point. Your DD won't miss out if you are reading to her daily and talking to her a lot.
Home education isn't standing in front of the room for 5 hours, but leading your DC in learning through play, reading, counting steps as you walk etc etc. She's still so young.
I do think that it needs to be led by the vulnerable family members. If they want to be with DD then I think that is more important than the small risk Google your area for what the risk is of your DD firstly contracting Covid and then transmitting it. I will be flamed for that, but if it were my family I think I would go for definites rather than maybes. By that I mean: the definite of the strong relationship, Vs the small maybe of illness.
Not everyone who goes to school is going to get Covid or even be exposed to Covid. Chances are much higher of illness and death from other causes, and we all weigh up those risks everyday as best we can, but not by locking ourselves up forever.
Not trying to be rude, OP, and I think you are one in a long list of people who are trying to work out this life puzzle in situations with relations who had been on shielding list.