Piriton liquid however is what you need, ask the pharmacist for the child version, it helps to reduce the itching somewhat.
thisisyesterday has great advice about oats. I used those underwear/delicates muslin bags, if you happen to have those, they are perfect as you can then use it like a sponge to pour more water over it. Use a HUGE handful.
Add some camomile tea to the bath, one cup brewed for 10 mins (two teabags is ideal).
I was recommended bicarb of soda into the bath too, which I used for both mine (in the last month). I have heard mixed reports with some suggestions this makes the itching worse. However, both mine were in a bath twice a day for several days and their itching was minimal, so hard to say if we were lucky or if it helped. most certainly all of the things i used did not harm and did not make it worse.
Cool bath, not too warm as that increases the itching/more spots!
If she is happy in the bath, leave in there as long as you want (i found with both mine, they wanted to stay in for a good 30 mins, more if I had let them).
Piriton liquid however is what you need, ask the pharmacist for the child version, it helps to reduce the itching somewhat.
Loose clothing, night dress if she has one, or if your house is warm, a vest. No knickers if possible, or if not, very loose cotton ones.
Keep the house cool if you can, open a window for fresh cool air.
If she has long hair, and this is the very beginning, pop her hair in a pigtail/plait if you can, so that as more spots appear you won't have to brush her hair daily. I put DDs into two french plaits half way then pulled into a bunch. It stayed in for 8 days, and when it came out, no knots on top of scabs to try and get a comb through. I know that might not be possible for toddlers, hair not so long.
It is horrible, I feel for your DD and hope she overcomes the worst of it quickly.