Child Component

<template>

    <template if:true={opportunity}>
    {myAmount}
    </template>
</template>


import { LightningElement,api, wire } from 'lwc';
import { getRecord } from 'lightning/uiRecordApi';
const FIELDS = ['Opportunity.Name', 'Opportunity.Amount'];

export default class DemoOptyAmount extends LightningElement {
    @api recordId;
    @wire(getRecord, { recordId: '$recordId', fields: FIELDS })
    opportunity;
    get myAmount() {
        if (this.opportunity && this.opportunity.data) {
            return this.opportunity.data.fields.Amount.value;
        }
        return 0;
    }


}

Demo Opty Parent

<!-- Demo Opty Parent -->
<template>
    <lightning-input label="parent id" onchange={handleIdChange}></lightning-input>
    <c-demo-opty-amount record-id={parentid}></c-demo-opty-amount>
</template>

import { LightningElement, track } from 'lwc';

export default class DemoOptyParent extends LightningElement {
@track parentid ='';

    handleIdChange (event) {
        try {
            let evtvar = event.target.value;
            console.log(evtvar);
            this.parentid = evtvar;
        }
        catch (err){
            console.log(err);
        }
    }

}