For those with child addon problems. this worked for me. this comes from the heros at support.
12ed12um (Curse Support)
Sep 8, 10:42 AM PDT
I am sorry for all the problems you are having playing Minecraft on the Twitch Desktop Client
Some users have mentioned, when installing modpacks, not to interact with the client or the computer until the installation is complete, for some users this resolves all the issues they are experiencing.
We are planning to push out an update that hopes to solve these bugs in the Beta release of the client.
To opt into the Twitch Desktop Client Beta,
Let's try the following: