That's an immigration check, to head off child slavery and other truly dreadful crimes. They are quite use to kids playing up too, so you need not worry too much about that. Customs is intelligence led in Europe (and increasingly so in the USA these days), so it's unlikely to be that. But like the CBP USA the UK's Border Force is now the operational side of customs clearance, on behalf of HM Customs and Revenue, so they do wear the same uniform.