Now about this texting whilst horse riding, and riding them two abreast on the roads. Texting is a judgment call and would depend on what is around them at the time, the temperament of the animal, the experience of the rider.
Many of these youngsters on horseback will have been riding from the age of 4 or less and so bt the age of 15 could have had 11 years of regular experience in the saddle. The horses as well may likely be very calm animals which are very experienced on the road, and not easily spooked - remember, the reins are used to direct a horse, not steer it, and keeping close contact with the bit by tugging on the reins all the time will just wind the animal up. You can't realistically compare horse riding with driving or riding a vehicle.
Regarding riding two abreast, it is also normally done for practical reasons. One of the horses or riders may not be very experienced, and so having a more experienced animal shielding it from the passing traffic will help to keep it calm. It also helps on narrow roads to encourage passing cars to wait until it is safe to attempt an overtake - Highway code makes some mention of it.
Obviously it is unrealistic to say this is always going to be the case, but if I saw this on the road, I would be more concerned with the behaviour of those around the horses and riders than the riders themselves as people can at times be very impatient when they come across slow moving road users.