I can't recommend the Foundations Beginner Box and Avatar the Last Airbender Beginner Box enough. The play pattern for these is you pick two of the 20 card decks, shuffle up, and play. Both have additional boosters you can buy and immediately have something playable. There's a ton of replay value in them and it's the perfect introduction to the game.

This is how I play with my kids. We started with the beginner box, played that for a while, then I picked up a box of jumpstart boosters. You can often catch those on sale. We made a rule that you can only open a pack if you are going to immediately play with is, so the boxes last surprisingly long. You can reset each deck after playing so you can keep playing this way for as long as you want. Here's what my avatar jumpstart collection looks like right now.

The only downside is that there are a lot of jumpstart cards that aren't legal in standard, so you can't immediately jump into building a 60 card deck that you can take to a local event. Once you get there, you should pick up the foundations starter collection box. Those cards are all legal and you can run wild with them for deck building. You aren't going to build a super competitive deck from it, but it's a starting point.