Every major Chassidus in BP has a day camp for girls. Belz, Bobov (probably has 2 by now), Pupa, Kloizenberg (Bais Esther), Vien.
The best day camp imo is Bais Yaakov of Boro Park. They are very strict about following safety rules, since they are such a big camp and incorporate a (very slightly more) diverse group of people than other BP institutions.
If you want to send your kids to a backyard day camp, make sure they follow elementary safety procedures. (have hatzala’s number, counselors know Heimlich and CPR, take attendance regularly, put sunscreen on the kids, etc.)