Maven multi-module project site with javadocs

Question

I would like use Maven for creating site for my application. This is a multi-module app, the parent module is simple site module, and first child is a core of app, the second is a GUI (Swing).

I now use follow for parent pom.xml

<modules>
    <module>core</module>
    <module>kayako-desktop</module>
</modules>
<build>
    <plugins>
        <plugin>
            <artifactId>maven-site-plugin</artifactId>
            <version>2.2</version>
            <configuration>
                <locales>en</locales>
            </configuration>
        </plugin>
        <plugin>
            <artifactId>maven-javadoc-plugin</artifactId>
            <version>2.7</version>
            <executions>
                <execution>
                    <phase>install</phase>
                    <goals>
                        <goal>aggregate</goal>
                    </goals>
                </execution>
            </executions>
        </plugin>
    </plugins>
</build>
<reporting>
    <plugins>
        <plugin>
            <artifactId>maven-javadoc-plugin</artifactId>
            <version>2.7</version>
            <configuration>
                <aggregate>true</aggregate>
            </configuration>
        </plugin>
        <plugin>
            <artifactId>maven-changes-plugin</artifactId>
            <version>2.4</version>
        </plugin>
    </plugins>
</reporting>

My core's pom:

<build>
    <plugins>
        <plugin>
            <artifactId>maven-javadoc-plugin</artifactId>
            <version>2.7</version>
            <executions>
                <execution>
                    <phase>package</phase>
                    <id>attach-javadocs</id>
                    <goals>
                        <goal>jar</goal>
                        <goal>javadoc</goal>
                    </goals>
                </execution>

            </executions>
        </plugin>
    </plugins>
</build>
<reporting>
    <plugins>
        <plugin>
            <artifactId>maven-javadoc-plugin</artifactId>
            <version>2.7</version>
            <configuration>
                <links>
                    <link>http://download.oracle.com/javase/6/docs/api/</link>
                </links>    
            </configuration>
        </plugin>    
    </plugins>

</reporting>

(I stripped out unrelated parts from both)

The problem: I tried mvn site:stage, but javadoc is not collected from core module. What do I wrong?