A 20-pin connector called Expansion Header is provided on the right side of the starter kit when viewed from the front. It is convenient to handle it if you make a child board that can be connected here.
The pin assignment of this Expansion Header differs depending on the starter kit, so please check the expansion header pin assignment of the starter kit you are using before proceeding. (Regarding how to obtainthe circuit diagram and parts list for the starter kit)

The board size and connector positions are shown in the figure below.

For more information, please refer to Application Note AN0031 “Making Prototyping Boards for the EFM32 kits”. This application note also contains information on the child board for the evaluation kit (development kit).
Please also check the SiliconLabs community site.