The new iPad Pro models for 2018 feature a large number of changes, but perhaps the most surprising is the switch from a Lightning to a USB-C port. On the one hand, this could be a pain for those who have a lot of Lightning-based accessories that won't work any more. But on the other, this means you can charge your iPhone from your iPad, which is an excellent bonus.
iPads have larger battery capacities than iPhone, but more importantly they tend to see less continuous use through the day: there's a good chance that a worker on the go will still have charge left over in their iPad when the iPhone gives up the ghost.
To charge your iPhone, you simply need to get the right cable (preferably Lightning to USB-C, although you may be able to use two cables and an adapter) and plug it directly into the iPad Pro. Provided the iPad still has charge, it will start transferring this to the iPhone. Check for a lightning bolt icon on the battery icon at the top right: this indicates that it's charging.
Neither the iPhones nor the iPads come with a Lightning-to-USB-C cable, so you'll need to buy one separately. The safest place is from Apple itself: the firm's USB-C to Lightning Cable (1m) is reasonably affordable at £19 in the UK or $19 in the US. (If you're flush with cash, there's a 2m version that costs £35/$35.)
Most importantly, this cable will definitely work. If you want to find a third-party alternative elsewhere (and there are definitely cheaper options), you should check with the seller that it's compatible with the iPad Pro; similarly if you're planning to use a third-party cable make sure you test it out before embarking on that important work trip.
Apple has warned that some USB-C accessories won't work and (understandably) cannot guarantee the effectiveness of third-party charging cables. Our suspicion is that for charging rather than data transfer, you should be fine with a cheaper alternative, but check first if you can.