When traveling by bus, you must remember that they have to travel in a child restraint system approved for height and weight . However, very few buses currently allow this.
In Spain for example, legislation does not stipulate how children under the age of 3 should travel, however the best option would be to install their CRS on the bus seat. Unfortunately this is not always possible, particularly if the bus does not have seat belts or if they are two-point belts.
All other passengers should duly fasten their seat belts if the bus has them. There are buses without this important safety system, especially when these are old.
If the bus you are traveling in does have seat belts, you need to know the difference between two and three-point seat belts. The two point seat belt does not affect children, that is to say, they are used for both adults and for very young children since they are fastened below the waist, on the pelvis. However, this is not particularly safe for children under 6 years old and it offers far less protection than a three-point belt.
On the other hand, a three-point belt may not be correctly adjusted and we will then need to use a booster seat. Three-point belts are designed for adults. This is why children need boosters, preferably with backrests, so that the seat belt does not cause injury and is well secured. The shoulder belt can press against the neck.