//tabs page html
<ion-tabs>
    <ion-tab-bar slot="bottom">
        <ion-tab-button tab="home">
            <ion-icon name="home-outline"></ion-icon>
            <ion-label>Home</ion-label>
        </ion-tab-button>
        <ion-tab-button tab="cart">
            <ion-fab>{{numCartItems}}</ion-fab>
                <ion-icon name="cart-outline"></ion-icon>
            <ion-label>Cart</ion-label>
        </ion-tab-button>        
    </ion-tab-bar>
</ion-tabs>

//tabs routing module ts
import { NgModule } from '@angular/core';
import { Routes, RouterModule } from '@angular/router';

import { TabsPage } from './tabs.page';

const routes: Routes = [
  {
    path: 'tabs',
    component: TabsPage,
    children:[
      {
        path: 'home',
        loadChildren: () => import('./home/home.module').then(m=> m.HomePageModule)
      },
      {
        path: 'cart',
        loadChildren: () => import('./cart/cart.module').then( m => m.CartPageModule)
      }
    ]
  },
  {
    path: '',
    redirectTo: '/tabs/home',
    pathMatch: 'full'
  },
];

@NgModule({
  imports: [RouterModule.forChild(routes)],
  exports: [RouterModule],
})
export class TabsPageRoutingModule {}


//Additional code
//tabs page ts
import { Component, OnInit } from '@angular/core';
import { SubscriptionCartOrganiserService } from '../services/SubscriptionCartOrganiserService';

@Component({
  selector: 'app-tabs',
  templateUrl: './tabs.page.html',
  styleUrls: ['./tabs.page.scss'],
})
export class TabsPage implements OnInit {

  numCartItems : number = 0;
  constructor(private cartManager : SubscriptionCartOrganiserService) {
    this.numCartItems = cartManager.getNumberOfItemsInCart();
    
    cartManager.cartProductsNumberDS.subscribe(num => {
      this.numCartItems = num;
    });
   }

  ngOnInit() {
  }
}