I made an example where the Grid is initially not bound and it is binding on buttons click and it is expanding all child Grids.
The example is working as expected on my end:
I can assume that in the real scenario the pageSize for the parent Grid is either not set or set to a larger number. Please have in mind that we recommend setting smaller page sizes for the parent Grid. This is required as otherwise, it will create a scenario where 15 or 20+ Grid has to be initialized at the same time and this is very resource consuming operation as it will create thousands of DOM elements, and then the performance will mainly depend on the browser and the machine: