If you look at the methodology
According to the CRC Committee, New Zealand (158th), the United Kingdom (156th) Italy (83rd) and Luxembourg (56th), for example,
could do more to improve the enabling environment they have built for children’s rights. These wealthy countries should be able to
invest more in children’s rights, but fail to do so sufficiently.
Thailand (8th) and Tunisia (9th) on the other hand deserve honourable mentions. These countries rank relatively high compared to their
economic status, as they do exceptionally well in cultivating an enabling environment for child rights. Thailand for examples scores
‘good’ on the enabling legislation for children’s rights. In the 2017 ranking Thailand even climbed from rank 21 in the KidsRights Index
2016 to rank 8, especially owing to improved scores on primary and secondary school participation in domain 3 ‘Education). Tunisia
scores well on domain 5 ‘the enabling environment for child rights. The country also has a low adolescent births rate, therefore scoring
relatively high on domain 4 ‘Protection’ (rank 22).
Essentially if you start out rich and high you must then improve or fall.
The UK and Italy as described basically didn't improve enough. This index isn't a measure of children's actual rights it is a measure of improvements of children's rights weighted by the wealth of the country.
So essentially the UK, New Zealand and Italy didn't implement enough new rights measured against what the UN expects.
It is saying if I got B last year and did the same amount of work and the same exam score this year I get F-. If I got a D last year and score 10% more this year I get an A.