Are there any halachos to keep in mind when opening up a Gan for children in an area where there might already be another gan or 2 found (especially if it’s in a building close by, how close, etc.) Is there any problems that might come up?
If you live in the area, there is no halachic problem of opening another Gan, provided that there is a sufficient pool of children for all the ganim to remain open even after yours will be functional.
The provision is based on the ruling of the Aviasaf, which is cited by the Rema and by later poskim such as the Chasam Sofer (Choshen Mishpat 79), the Beis Efraim, and other authorities.
If you don’t live in the area, there is a problem of opening the extra gan if the current ganim object to it.