Last summer, Sonya*, aged 17 at the time, had endured more than two years of a difficult life under Russian occupation in the Kherson region of southern Ukraine. Her foster mother agreed that she needed a break.Growing up under forced assimilation had been scary, Sonya says, and her Russian-controlled school had offered to take her to a holiday camp in Crimea, a balmy peninsula once famed for being the spa of the Soviet Union.Children at an Avangard camp. The UK imposed sanctions on the organisation over the forced deportation and brainwashing of Ukrainian children. Photograph: Joe Plimmer/TelegramIt was only after they had left that she discovered they were actually being taken to Volgograd, a city more than 600 miles (1,000km) away in the south-west of Russia. Far from being a holiday resort, the city is home to one of a growing number of state-run military-style training camps held for Ukrainian and Russian children.Sonya spent a month at the Avangard camp, which has several other branches across Russia, and had sanctions imposed on it by the UK in November for its role in the forced deportation and brainwashing of Ukrainian children. She says she and other children faced a programme of activities that included being taught to dig trenches and rig them with booby traps and trip wires, load machine guns, take part in military-style formations and carry other children on their backs to simulate medical evacuations.Similar to Soviet-era youth camps, these centres have escalated in number since the start of the full-scale invasion of Ukraine in 2022. Critics say they are used to indoctrinate children to prepare them for future service in the Russian military.