Natasha and Clint gave each of them a dagger and pocket knife incase that they couldn't use their powers/magic to get out a of a situation or just to threaten someone.

Loki had put magic on their new daggers and pocket knives so that they would always return to them, no matter how far they had thrown them. He also gave each of the a ring that kept them safe from any minimal damage causing curse, hex, jinx, charm or potion (as he didn't fully know what spells and potios they used in the mortal world).

Tony and Bruce gave them each a smart watch with their own AI (in Hades case he just added Merlin to it). Pietro had names his Speedy and Wanda had names hers Scarlet. Loki had also made them (and Hadrian's glasses and walking stick) magic proof. 

Steve and Bucky gave them notebooks, pens, pencils, erasers, highlighters and rulers and without Steve looking, Bucky gave them each another pen, that wasn't a pen. If you took the cap off, you could see a blade instead of the actual pen. Wanda's was red, Pietro's was blue and silver and Hadrian's was black, silver and gold. A knife/dagger disguised as a pen, clever.

Thor gave them the birthday cake, made by Frigga herself. 

After all the presents had been opened, a tap on the window was heard.

"I knew it! I knew it! I knew it!" Hades started to shouted and jump in excitement, "It's an owl isn't it?"

"Yes it is," Regalus smiled as he opened the window and the owl flew onto his shoulder. He took the letters from him and gave one to each of the children. 

Hadrian felt the the wax in the envelope and grinned like a mad man. "They're our Hogwarts letters right? Aren't them Dad? One for each of us, just how I told you!"

Wanda and Pietro squealed and rushed to their brother's side, all three of them jumping and laughing together. They then opened their letters reading them out loud for Hadrian to know what they said.

{the following is copy pasted from harrypotter.fandom.com}

________________________

HOGWARTS SCHOOL of  WITCHCRAFT and  WIZARDRY
(Headmaster Albus Dumbledore, Order of Merlin, First Class, Grand Sorcerer, Chief Warlock)

Dear Mr Hadrian,

We are pleased to inform you that you have been accepted at Hogwarts School of Witchcraft and Wizardy. Please find enclosed a list of all necessary books and equipment. Term begins on 1 September. We await your owl by no later than 31 July.

Yours sincerely,

Minerva McGonagall
Deputy Headministress
________________________

UNIFORM

First-year students will require:

1. Three sets of plain work robes (black)
2. One plain pointed hat (black) for day wear
3. One pair of protective gloves (dragon hide or similar)
4. One winter cloak (black, with silver fastenings)
Please note that all pupil's clothes should carry name tags.

COURSE BOOKS

All students should have a copy of each of the following:

The Standard Book of Spells (Grade 1)
by Miranda Goshawk

A History of Magic
by Bathilda Bagshot

Magical Theory
by Adalbert Waffling

A Beginner's Guide to Transfiguration
by Emeric Switch

One Thousand Magical Herbs and Fungi
by Phyllida Spore

Magical Drafts and Potions
by Arsenius Jigger

Fantastic Beasts and Where to Find Them
by Newt Scamander

The Dark Forces: A Guide to Self-Protection
by Quentin Trimble

OTHER EQUIPMENT

1 wand
1 cauldron (pewter, standard size 2)
1 set glass or crystal phials
1 telescope
1 set brass scales 

Students may also bring, if they desire, an owl OR a cat OR a toad.

PARENTS ARE REMINDED THAT FIRST YEARS

ARE NOT ALLOWED THEIR OWN BROOMSTICK

Yours sincerely,Lucinda Thomsoncile-Pocus
Chief Attendant of Witchcraft Provisions
________________________

Wanda's and Pietro's said the same (except the name, duh). They looked at their parents with puppy eyes, "Can we please go? Please, please, pleas, plea-"

"I allow you, but your mother has to confirm it," Nat gave Reg a death glare that would make Salazar Slytherin proud of. 

"Yes, but you have to call us atleast once a week and the first night after you arrive. Okay?" she agreed.

The three kids hugged their parents (after helping Hades to do so first) and thanked them repeatedly. Meanwhile this happened, Loki had written back a response agreeing that they would go.

"Pack your bags everyone! We are going to London tomorrow!" Regalus exclaimed, a grin growing on his face.

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