Especially, the mother tongue. You can teach an extra language and I am writing for Urdu, because in many Indian states, children don't have the option to learn Urdu in schools.
The third language in schools is generally not Urdu. Hence, they are unable to learn it properly. It is time to sit with kids and teach them Urdu.
In Muslim households, Urdu words are known, grammar is known and it is spoken. What is needed is just to read the script and write. So for that, just a small effort is needed.
Barely 20-30 min a day, it doesn't take more. As most kids are familiar with the script too because they learn the Holy Quran [Arabic], it is even more easy. But it needs a bit of discipline and patience.
You have to do it each and every day. Within a couple of months, the kid would definitely pick up enough. I had written a similar post on this blog five years ago. Check the link below
READ: Teach kids an additional language or their mother tongue in vacations
Subscribe to children's magazines. Kids like them, make them read. You can start with a Urdu primer also. There are many Urdu magazines in India. NCPUL's Bacchon Ki Dunia, Hilal, Noor, Umang and Payam-e-Talim have good circulation.
LIST OF CHILDREN'S MAGAZINES IN URDU IN INDIA
1. Bachchon ki Dunia, Delhi
2. Umang, Delhi
3. Noor, Rampur
5. Naya Khilauna, Kolkata
7. Butul [focuses on girls, more]
9. Gul Bootay
11. Achha Saathi