I have 7 yo twin girls, they are, apparently, the only ones in their class still in high back boosters. Naturally I am taking this with a large pinch of salt, but am aware that they are the tallest in their class (90-98th centile) and finding their current seats very uncomfortable. We do have booster seats without backs for their nanny to use, but DT1 seems to get a bit travel sick without the 'ears' as she calls them, on longer journeys. So, any suggestions as to when to switch to booster only, or which hbb seat to get to accommodate tall girls? Thanks!