I paid 10HDB to join the game. Do I now own some cards? What are they worth?

You can check the current value of your Splinterlands card collection on the Discord server of Splinterlands. Mine is currently worth $846.15 USD.

If you do not use Discord, then you can also check it on PeakMonsters.

Realistically do I need to spend more money to get set up?

I started playing Splinterlands (Steem Monsters) on 2018.12.30. I also paid $10 USD in Steem for the Beta starter set. I played successfully without little to no further investment. Nowadays the situation could be different, but nowadays it is better to rent the cards than buy them, because nowadays they are very expensive to buy. This was very different, when I started playing it. You could have bought many of the cards for a few cents in 2018.

How do I earn SPT tokens and what's the best way to earn more. I heard there were airdrops. What do I need to do to qualify for them?

One way is to use the #spt tag on your posts. Another (passive) is to delegate SPT to @monster-curator. You will receive SPT daily.

Is the game child friendly or are there any risks?

Yes. It is child friendly. There is even an Android app to play the game. Anyone can play it.

Splinterlands is a fun, entertaining, enjoyable game. Nowadays it is one of my favorite games. Good luck and have fun.

I give you some !PIZZA.

Have a nice day. All the best. Greetings from Hungary.