You don't get stats broken down quite that cleanly.
UK passenger fatalities age 0-4 are (thankfully) so low that you can't calculate it from a single year because with those low numbers the data is too easily skewed by a single accident. You have to average it over several years. It seems to come down to about 6-7 fatalities per year in these age groups.
It does look like under-1 year olds are underrepresented in this data. Since under-1 year olds are significantly more likely to be rear facing in the UK than any other age group, it's likely that this is showing a protective effect of rear facing. If you look at the last 10 years of data (2013-2023) this is the pattern for fatalities broken down by age:
0 yo: 7
1 yo: 15
2yo: 14
3yo: 15
4yo: 8
It's possible that children aged 1-3 years are more vulnerable, but the other possibility is that children under 1 are likely to be in rear facing seats, and children over 4 are old enough to gain some protection from a seatbelt or booster seat, so children aged 1-3 are more represented because they are more likely to be in forward facing seats and/or in booster seats or seatbelts too early.
If you include serious injury then unfortunately, the numbers are much higher so it is more accurate to look at single years. It seems in recent years it's about 100 children each year 0-4 years who are seriously injured as passengers in car accidents.
Again, children under 1 are slightly underrepresented in the statistics although this is not as stark as in the fatality numbers. It's more like a difference of 30% than half.
However, if you go back to look at records from earlier years then you see a MUCH bigger difference in the numbers of under-1s vs 1yos who are seriously injured as car passengers. The tipping point is about 2016. Anything from 2016 and earlier, the number of seriously injured 1yos jumps to about twice the number of seriously injured children under 1. The data goes back to 2004 and while overall injuries are higher in all groups in 2004 (closer to 200), the pattern is the same there.
That's interesting because it seems to match almost exactly my observation that 2016 specifically was basically when ERF (in terms of rear facing into the second stage seat) became "mainstream" - spin seats were no longer new and novel, they were popular and affordable. The "123 seat" type which had dominated the market since the 2003 law change fell out of favour and spin seats took over. Joie launched basically all of their affordable (sub £100) ERF seats in 2015/2016. Stage 1 of R129 which introduced the 15 month min FF rule was released in 2013 so a lot of seats on the market in 2015/2016 were R129 compliant.
Yes, people in the UK still tend to forward face early on average, but that was the tipping point in my (mainly MN based) observation of it being seen as a normal thing to continue RF at least for a little while into the next stage seat rather than RF being exclusively for the baby stage and the switch made at ~9-12 months. I am surprised but pleased to see such a significant shift in the injury data.
Of course, the shift could be due to something different entirely - improvement in general in car seats (isofix was most likely an improvement for example), safety improvements in cars, medical advances in treating injuries sustained by children in car accidents - something else not thought of.
This is the dataset I'm using to get the numbers: department-for-transport.shinyapps.io/collision_analysis_tool/