Computed property based on child components

Is it possible to create a computed property that relies on child components data? Seems like a trivial task but I can't figure it out...

foo component

<template>
    {{ foo }}
</template>

<script>
export default { 
    computed: {
        foo() {
            return Math.random()
        }
    }
}
</script>

parent component

<template>
    foo computed property sum: {{ sum }}
    <Foo v-for="n in 10"></Foo>
</template>

export default { 
    computed: {
        sum() {
            // return...?            
        }
    }
}
</script>