There are always a lot of questions regarding the recommended size and weight for using child restraint systems. With the legislation in mind, we know that children who are 135 cm in height or shorter must travel on the rear seats using an approved child restraint system suitable for their size and weight.
However, the legislation does not take into account possible weight differences, particularly when it comes to young children. The percentile, for example, tells us that children of the same age have certain weight variations between two values: the minimum and the maximum.
A percentile is a measure used in statistics indicating the value below which a given percentage of observations in a group of observations falls. To use an example: the 60th percentile in weight means that 60% of boys, or girls, since there are separate tables per sex, weigh the same or less than the child in question, and 40% will weigh more.
Having said this, it is possible that the child is in a higher weight percentile and a lower size percentile, and vice versa. In such a case, the child is likely to be somewhat «chubby», while in the opposite case the child would be slimmer. There are numerous possible combinations, considering that in terms of percentiles, anything between the 3rd and the 97th percentile is completely normal.
Therefore, a child who has not reached 135 cm in height as stipulated by the law may be over 36 kilos in weight, although this is fairly unusual. In such a case, the child's parents would be unsure what to do about the child car seat since the recommendation is that the child continues to use a booster seat with a backrest until he or she is 150 cm tall. However, taller children within higher percentiles would easily exceed this weight before reaching 150 cm.
When traveling by car, the answer is always the same: children must use a booster seat with a backrest for as long as possible until they are able to use a seat belt. This seat belt must be properly positioned to avoid problems (take a look at this extensive information on how to properly position the seat belt here).
And what about the «limitation» referring to the maximum weight allowed? This is the maximum recommended weight. This means that the seat has not been tested with a higher weight than this and therefore the manufacturer cannot guarantee the same level of safety as for a child whose weight is within the established limits. Does this mean that the child seat is not safe? No it does not. Although this may seem like a contradiction, if we stop and think about it, a child car seat will not go from protecting a child at 36 kilos to becoming unsafe for a child weighing a kilo more.
We ought to be aware that under these conditions the manufacturer will not assume liability for the seat, and will not certify that the booster seat's protection will be optimal since there is no legislation to verify this, nor any tests under such conditions.