My Friend Peppa Pig

Once completing all the achievements, go back to Peppa's House and go up to the bedroom and the cut scene for potato city will start.

Edit. This achievement is known to be buggy as the cutscene will not always trigger

Eventually after completing all other Achievements you'll head back to your house and Daddy & Mommy Pig will take you to Potato City as a reward and the Achievement will unlock.
I show this in my Achievement Walkthrough starting at 40:31 minutes into the video.

This is the final achievement. After visiting all of the areas and getting all the achievements. You can go back to Peppa's house and go upstairs. This will trigger the final cutscene and unlock the achievement!

VIDEO GUIDE:

Potato City is available when you visit the areas and complete the missions there. However, if going back to the room does not trigger the cutscene, leave Peppa's room and go downstairs. At the side of the stairs, you will see two pictures posted on the wall. The one on the left is the one for Potato City. Go to that and interact with the photo and it will trigger the Potato City scene and net you the achievement. The other photo triggers the Windy Castle visit.

The above video will net you all achievements in just under 50 minutes with ease. I left out commentary as some children may enjoy just watching the playthrough without commentary!

Below is a full text guide for the entire game, the "Always Remember To Eat Your Five" achievement is the very last paragraph and is in bold for anyone looking for specific help with that achievement. It should be the last achievement you encounter on your playthrough.

Where's Mr Dinosaur? - To achieve this you need to find Mr Dinosaur for George. As soon as the game starts, interact with the TV and he should run in to the room crying. After the cutscene, go outside to find Mr Dinosaur stuck in a tree. Go back inside to get your dad and have him retrieve the dinosaur.

Daddy's Glasses - After rescuing Mr Dinosaur, go back inside and interact with the TV a few times. After doing this you want to sit down on the sofa, and your dad should walk in missing his glasses. You can find the glasses in one of three locations - either by the front door on the floor, on the kitchen table, or upstairs.

Rounding Up The Chickens - After leaving your parents house, head all the way left to your grandparents. Enter the living room and interact with the food bowl. Head to the kitchen to get the bird seed and fill up the bowl. Your grandma will come in and task you with finding and returning the chickens to their coop. The first chicken will be in the living room, whilst the other two will be outside - you'll see them both on the way to the coop, which is outside and to the far left.

What's In These Rockpools? - After rounding up the chickens, interact with your grandparents car and head to the beach. Once there, just interact with the rockpool three times for this achievement.

Whose Tracks Are These? - After getting back from the beach, head back to your parents house and get on your bike. Cycle right to The Forest. After cleaning up the trash on the floor, head right and you'll see a trail on the floor. Interact with and then follow it to the right until you find the source of the tracks.

Collecting Sticks For The Campfire - After following the ant trail and having a picnic with your family, the school bus will turn up. You'll be tasked with collecting sticks for the campfire. Simply follow Peppa and pick up the sticks in clear view, and a further two sticks one scene over to the left.

The Best Snowman Ever! - After finishing your camping trip, head to the left and interact with the hot air balloon. Upon arrival, interact with the ski lift. Once at the top of the slopes, interact with the snowman. You'll be tasked with building a snowman of your own. Simply interact with the large ball of snow, followed by the smaller ball of snow, and then the 3 items for the snowman's face.

The View From Windy Castle - Upon returning from the ski slopes, head back home. Enter your house and your parents will suggest you go to Windy Castle. Upon arrival, you'll enter the castle and automatically use the telescope. It should pop straight away, but if not just scroll through the various scenes as shown in the video.

Helping Tiddles The Tortoise - As soon as you arrive back home from Windy Castle, get back in your parents car and head left. Once at the school, head one scene right and interact with the tree. Tiddles the Tortoise will be trapped and in need of help. Head back to the school and tell your teacher, who will join you at the tree. After the cutscene, head left again to retrieve the ladder propped up against the school.

Rocket To The Moon - After saving Tiddles, get in the blue school bus and you'll head to the museum. Once inside, head all the way to the right and you will automatically ride the rocket for this achievement.

Always Remember To Eat Your Five! - After riding the rocket, head right to leave the museum, and then interact with the hot air balloon. After the cutscene, get on your bike and cycle back home. Enter your house and go upstairs, and your parents will take you to Potato City. After your first cutscene here your final achievement will unlock.

After visiting all areas and unlocking all other achievements, make your way back to Peppa's house and enter her bedroom. The cutscene will then begin, and the achievement will pop once you arrive. Oink!