You may use the following type_traits:
template <typename Base, typename T, typename... Ts>
struct are_base_of :
std::conditional<std::is_base_of<Base, T>::value, are_base_of<Base, Ts...>,
std::false_type>::type
{};
template <typename Base, typename T>
struct are_base_of<Base, T> : std::is_base_of<Base, T> {};
And then use
template <typename... Ts>
typename std::enable_if<are_base_of<BaseClass, Ts...>::value, void>::type
function() {
// do stuff with Ts, which is ensured to be a child class of BaseClass
}