Two children sharing one seat?

[psb777]

In several jurisdictions, definitely Australia and I think the USA and maybe NZ, it is legal for two children to share one passenger seat using one lap belt. There is a weight limit, total 77kg or 170 lb, I think. Is there a similar provision in the UK or elsewhere in Europe?

[WestWind1950]

the regulation in Germany:

§ 19 LuftBO
Ergänzungsausrüstung, die durch den Verwendungszweck erforderlich ist
(1) Luftfahrzeuge, die für die Beförderung von Personen oder Sachen verwendet werden, müssen ausgerüstet sein mit:

1. Einem Sitz für jede Person und einem Anschnallgurt für jeden Sitz; zwei Kinder mit einem Höchstalter bis zu zwei Jahren oder ein Kind mit einem Höchstalter bis zu zwei Jahren und ein Erwachsener können auf einem Sitz untergebracht werden; in Flugzeugen, die nicht in der Lufttüchtigkeitsgruppe Verkehrsflugzeuge zugelassen sind, und sonstigen Luftfahrzeugen mit einem höchstzulässigen Fluggewicht bis zu 5.700 kg können zwei Kinder mit einem Höchstalter bis zu 10 Jahren auf einem Sitz untergebracht werden, wenn dadurch die Sicherheit und Ordnung nicht gefährdet wird; Freiballone sind von den Vorschriften dieser Nummer ausgenommen;

roughly translated:
there needs to be a seat belt for each seat. 2 children of max 2 years or one child up to 2 yrs. and an adult may share a seat; in air transport aircraft and other aircraft of max. 5,700 kg, 2 children up to max. 10 yrs. of age may share one seat as long as the safety isn't impared. The ruling doesn't apply to manned balloons.

[Pilot DAR]

The only aircraft I know of where it is intended that two persons share a seat belt is th Piper Supercriuser. The fact that this was approved for that aircraft does not make it a good idea. Most aircraft have a specified limitation of the number of occupants, which with the foregoing exception, will equal the number of seatbelts. They are also insured for a maximum number of passengers. Two good reasons to not exceed the passnger number regardless of weight.

Let along legality, for the sake of safety I would simply never do this.

In the case where Airlines permit an infant to occupy the same seat as a parent, a second seatbelt is provided to independantly secure the infant to the partent's seatbelt without being within it. A parent just holding the infant is a bad idea. Let's say the infant weighs 20 pounds, and were in the recent Turkish airlines crash in Amsterdam. That plane stopped at somewhere around 20G forward deleration. During that time, the 20 pound infant now imposed a 400 pound force to be restrained. Could the parent hold that load in the best of times, let alone during a crash?

Pilot DAR

[B2N2]

It is NOT allowed in the USA.

(3) Except as provided in this paragraph, each person on board a U.S.-registered civil aircraft (except a free balloon that incorporates a basket or gondola or an airship type certificated before November 2, 1987) must occupy an approved seat or berth with a safety belt and, if installed, shoulder harness, properly secured about him or her during movement on the surface, takeoff, and landing. For seaplane and float equipped rotorcraft operations during movement on the surface, the person pushing off the seaplane or rotorcraft from the dock and the person mooring the seaplane or rotorcraft at the dock are excepted from the preceding seating and safety belt requirements. Notwithstanding the preceding requirements of this paragraph, a person may:

(i) Be held by an adult who is occupying an approved seat or berth, provided that the person being held has not reached his or her second birthday and does not occupy or use any restraining device;

and:

(13) An approved safety belt with an approved metal-to-metal latching device for each occupant 2 years of age or older.

[goatface]

PSB

You don't make it clear whether you're thinking about putting two children in one seat or just making a general enquiry, but the consequences of something as inocuous as a rejected take off at speed or, god forbid, a forced landing or an incident incurring similar G forces, don't bear thinking about.
At the sort of age you are discussing, children's bones and skulls are incredibly vunerable, which is probably why the practice is banned is some countries and severly restricted in others.