It depends mainly on how much conductive material is in the wall. I think the transmission power is sufficient, after all, in our case the signal reaches about 15 m through three interior walls and the exterior wall, or about 15 m through an interior wall, a thick concrete floor and another interior wall into the laundry room. We live in a rented flat, new building.
Keep in mind that the higher the transmission power, the higher the radiation exposure of the baby monitor.
In my opinion, there are also baby monitors that can transmit directly to the mobile phone via WLan. With this, you could amplify the signal outside the children's room with a repeater so that you have sufficient reception in the garden.

If you want to know exactly, you may find the answer here: PDF of a PowerPoint presentation "Electromagnetic waves in matter" from the University of Kassel --> https://www.uni-kassel.de/fb10...