The O.V. chip card is great and mine is still good for another year . Planning to get one also for my d-i-l for our trip to The Netherlands this June 2019 . Her 2 children ( my grandkids ) are 11 years old twins ; Are there cards for children that are just as convenient as the O.V. card for adults ?
I'm assuming you have an "anonymous/disposable" OV chipkaart and not the personal card (you're unlikely to have a personal chipkaart: you would have had to submit a photo and have access to a Dutch bank account, as the personal cards are geared toward Dutch residents).
In that case, there's no difference between adult and child fares or cards, since discounts for children and other age-related discounts are only available on personal chipkaarts. So you can just get two additional anonymous chipkaarts for your two children.
If you do happen to have a personal chipkaart there are some discounts for children.
The only other thing I'd mention is that the Amsterdam day ticket for children ages 4-11 does exist and costs €3.75, in case that fits your use case.
In case your grandchildren don't have their next birthday too soon: under-12s can travel on so-called Railrunner tickets for EUR 2,50 per day. Otherwise it is indeed their own OV Chipkaart.
Thank you Ton ! The twins turn 12 at the end of 8/18 so we are able to buy the Railrunner ticket for them . Seems like a good deal .
I forgot to mention that, as the name implies, Railrunner tickets are only good on the trains.
Twins turn 12 in august of 2019 ! Sorry