AppForKidsDesktop/AppForKids/Registration.axaml

<Window xmlns="https://github.com/avaloniaui"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        mc:Ignorable="d" d:DesignWidth="800" d:DesignHeight="450"
        x:Class="AppForKids.Registration"
        Title="Registration">
    <StackPanel Spacing="15" HorizontalAlignment="Center" VerticalAlignment="Center">
        <TextBlock Text="Введите логин"/>
        <TextBox Width="300" x:Name="LoginTextBox" ></TextBox>
        <TextBlock Text="Введите пароль"/>
        <TextBox Width="300" x:Name="PasswordTextBox" ></TextBox>
        <StackPanel Spacing="5" Orientation="Horizontal">
            <Button x:Name="RegisterClick" Click="RegisterClick_OnClick" Content="Зарегистрироваться"/>
            <Button x:Name="LoginClick" Click="LoginClick_OnClick" Content="Войти"/>
        </StackPanel>
    </StackPanel>
</Window>