To answer the specific question here, Heathrow Airport Limited (HAL) do offer free wifi to all passengers for the length of their time at the airport. However it is necessary to register with HAL at first log in. I think it's fairly intuitive but there are multiple questions asked, unfortunately. The bigger problem is that HAL deliberately keep the wifi speed down since they charge for the higher network speeds of 10-20 Mb/s. So I'm not sure video calling is a good fallback, and in any case if your parents aren't that good with technology you may be better off looking at SIM card options. The other option is that multi wifi providers such as Boingo can use the HAL service, so that avoids registration.

However this is all an unnecessary counsel of despair! Heathrow is a large and complex airport, the management very much know that, and so more than probably any other airport I have been to, a lot of thought has been put into pictorial signs, which are clear and non language specific. Thousands and thousands of non English speakers successfully transfer at Heathrow every single day without issue. It's harder being a parent than navigating Heathrow so I wouldn't underestimate their abilities here.

If you wanted a simple guide for them, tell them after the Prague flight (presumably on British Airways) to look for the purple Signs for Flight Connections. They don't want baggage reclaim (yellow signs). They go through security there, no avoiding that, it's a clear corridor process, and then they are placed into the central shopping area of Terminal 3. The gates are signed all over the place there, and in the unlikely event of confusion just find someone in a purple uniform for help - by showing their boarding passes and pointing at the gate number I'm sure they will be given hand signal as to the direction to go. And why a purple uniform? Because someone worked out that a consistent colour scheme would help passengers looking at purple signs, and that thinking pervades the building. If they are Czech or Slovak speakers, I happen to know that some of these staff speak those languages.

If the onward service is AA then there's further advice I need to give, but if it's BA then that's really all they need to know.