@Lundeborg 

What is the current network diagram? How do you connect these devices together? 

With the device in childrens room connected to the Deco network, please click here to verify its wireless negotiation speed. 

Then try to reboot the whole system and let the device rescan the wireless network and connect to the closer Deco unit.

May it help and have a good day. 

@Lundeborg 

So the current network diagram is modem--main Deco )))((( slave Deco, right? 

The Deco app does not have the option to choose channel manually, while we have received the same request and reminded our R&D team.

Back to your case, have you ever tested the wireless negotiation speed of your device in children's room?

https://www.tp-link.com/support/faq/2265/

Besides, try to turn off the main Deco and turn on the 2nd Deco only and ensure that the device in the children's room has connected to the slave Deco, then turn on the main Deco.

After that, please verify the speed it can get. 

Or you can try to swap the locations of the 2 Decos. https://www.tp-link.com/support/faq/2400/

Good day.