Yes, they can still buy the Children Co-Travel Card at the counter. It costs CHF 30 per child, and is still the cheaper option than buying Child day passes or Child tickets. The Co-Travelcard for tourists cannot be bought online and is not mentioned on the SBB website. The link I pasted above is about the resident offer, just to give you an idea.
The Swiss Travel Pass comes with the Swiss Family Card but this is for parents only, not for other relatives.
They should go to the counter after arrival and ask for Children Co-Travelcards; there is really no issue or any strings attached; as foreigners, they will receive them as paper tickets and the children can travel free of charge where ever they travel together.
There is no similar product available online.
Alternatively, they could buy the Children’s Day Pass for CHF 19 / day & child (note: the Co-Travel card costs CHF 30 for their entire stay). However, the day pass is not valid for mountain excursions while the Co-Travelcard is.