I know that Khala is Mahram to me, and her daughters are non-mahram for me. We have a huge age gap (9-10 years), if they have reached the age of puberty, can i still play with them or have chat with them or should I keep a gap between from now on? Does chatting on whatsapp is also considered haram?

Ustadh AbdulAziz al-Haqqan

Once she has reached puberty she is a non-mahram woman, so the playing and joking stop, and what remains is a respectful greeting and speech when there is a need, with no seclusion. WhatsApp is the same, so no messaging.