any one live in denmark? car seat query

[UniS]

We are heading over to denmark in teh summer. We are arriving by plane but will be using grandparents car while we are there . As I understand it danish law on child seat belts is
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The use of fitted seat belts is compulsory for persons over 3 years of age.

Children under 3 years of age must be secured with approved safety equipment appropriate to the child's age, size and weight.

Children from 3 to 6 years of age may use approved child seat or booster seat instead of seat belts.
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My son is heavey enough to use a 15kg+ booster desinged to be used with adult seat belt but is under 3. WIll use of a booster plus adult belt be OK, or do I have to work out how to get a 5 point harness seat for him to use.
Anyone living in dk wish to comment?

actually even though they say from 15kg they are NOT reccomended until at least 22kg, even better 25 kg

until 22 / 25kg it is recommended at the very least they have a high backed booster seat (NOT a booster cushion)

there is absolutely no way i would put a 2 year old on a booster cushion... they should ideally be in a 5 point harness untl 18kg anyway... much safer than being restrained by an adult belt, even if adult eblt used with a booster chair.

booster cushions offer excatly no side impact protection which in a crash the younger bones need even more than older childrens bones, and are not very comfortable for sleeping either as child would just flop off