public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();

        this.dateRangePicker.Loaded += DateRangePicker_Loaded;
    }

    private void DateRangePicker_Loaded(object sender, RoutedEventArgs e)
    {
        DateRangeMaskedInput dateRangeMaskedInput = this.dateRangePicker.FindChildByType<DateRangeMaskedInput>();

        if (dateRangeMaskedInput != null)
        {
            dateRangeMaskedInput.ValueChanging += DateRangeMaskedInput_ValueChanging;
        }
    }

    private void DateRangeMaskedInput_ValueChanging(object? sender, Telerik.Windows.Controls.MaskedInput.RadMaskedInputValueChangingEventArgs e)
    {
        if (e.NewValue.ToString().Any(char.IsLetter))
        {
            e.Handled = true;
        }
    }
}