I'm using a simple
Yes, the parent will be able to read all the data. To put your mind at ease, try the following in a shell:
echo test | (sleep 1; cat)
The echo command is done immediately; the other side of the pipe will wait one second and then tries to read from it. This just works.
The child can also write more than 64 kiB without problems, as long as the parent will keep on reading in a loop, although then it won't be atomic any longer.