I created a parent race with sub-races, but when I go to create a character with that race, each sub-race shows up as its own race rather than as a drop down like the elves and dwarves do. 

Please see the Subclasses & Races FAQ #5: (https://www.dndbeyond.com/forums/dungeons-dragons-discussion/homebrew-house-rules/131411-a-homebrewers-how-to-faq).

Well that sucks. Thanks

This is one of the reasons why I no longer use homebrew sub-races; I've switched over to structuring my "sub-races" as options within a single race entry, which interestingly seems to be the trend now in Mordenkainen's Monsters of the Multiverse anyway (lots of things that used to be sub-races are now just options, e.g- Aasimar types).

I made a guide to try and describe how I do this, in case you think it might be something you'd like to try instead. It has a few minor drawbacks of its own, but personally I find it a lot cleaner to work with.

Yup, I did the same thing.

That's great, but what if you have a sub race that has spells and a choice to use Int, Wis or Cha.

Quote from Zagoradragongod >>

That's great, but what if you have a sub race that has spells and a choice to use Int, Wis or Cha.

 can do something like 3M Kobold did.

Quote from Zagoradragongod >>

That's great, but what if you have a sub race that has spells and a choice to use Int, Wis or Cha.

You'll need to do three options for that sub-race, one for each casting type. For example, "Mistborn (Intelligence)", "Mistborn (Wisdom)", "Mistborn (Charism)" or whatever your sub-race is called.

If you're using a separate trait for the details page then you still only need one entry for the sub-race there, describing it like normal, but for the builder it'll need to be three choices, so that does mean a bit more clutter in the list, but shouldn't be a problem with only one or two sub-races with casting.

If you have more sub-races with casting, there is a way to narrow the number of choices in the list but I won't describe it now as it's a little complex and you may not need it. It's disappointing that DDB hasn't given us an easier way to do this, e.g- a modifier that can override spellcasting ability ("Choose Intelligence, Wisdom or Charisma") or such, as it would make their lives easier as well as ours.

Thanks, I have a lot of sub-races unfortunately.

Quote from Zagoradragongod >>

Thanks, I have a lot of sub-races unfortunately.

Do many of them have spellcasting?

The alternative way to structure it is that you have one trait called say… "Sub Race Type" which would have four options "Spellcasting (Intelligence)", "Spellcasting (Wisdom)", "Spellcasting (Charisma)" and "No spellcasting" (if some of your sub-races don't have spellcasting).

You can then make the options under your Sub Race feature dependant upon the choice of that one (set them as a prerequisites). You still need one copy of each sub-race for each spellcasting type, but this lets the player narrow down the options, because only the INT/WIS/CHA/none versions of the sub-race(s) will appear in the list after that first choice is made.

That said, you don't need to do this up front, you can add it later once you're happy everything is added.

You might also look at ways you can simplify the sub-races; for example, I recently made a "Custom Tiefling" race which is basically just all of the official Tiefling sub-races and variants combined into one race with a few options; the type of ability score increase, the type of damage resistance (cold or fire) and then spellcasting or winged. Spellcasting has versions for each ability (INT/WIS/CHA) and each resistance type for six total, but because they're tied to the resistance type you only see the three you need (plus winged which both types can take). I got it that simple because I decided not to use an option for each of the old sub-races as once you removed fixed ability scores the only difference was the racial spells, so I decided to just give them the same three spell entries (cantrip, one level 1 cast at level 2 and one level 2) and then let the player pick which spell they want for each, which massively simplified it (and enabled some new combos as well, my Tiefling bard who never used darkness now has burning hands instead, since he can't get a lot fire damage spells otherwise).

Thought I'd mention it in case you're doing anything similar to that, as I could try to show you how I did it if it is.

sounds good. many of the spells they cast are utility spells, so I don't worry aobut spellcasting ability with those. Is there a way to do custom  classes. Not just sub classes.

Quote from Zagoradragongod >>

sounds good. many of the spells they cast are utility spells, so I don't worry aobut spellcasting ability with those. Is there a way to do custom  classes. Not just sub classes.

Unfortunately not, we can only create custom sub-classes on D&D Beyond. That said, most of the time there's no reason to make a custom class as it's a lot of work so you need to be sure you're making something genuinely new and necessary; depending upon what you're trying to capture, a sub-class is usually enough.

However, if you want to create unrestricted homebrew you might consider homebrewery; it's basically an online text editor that lets you create official-looking page layouts like you'd find in an actual printed book. It has a bit of a learning curve if you've never used markdown formatting (same as is used on wikis and similar) but the starting document shows you everything.

Obviously it won't integrate with the character sheet on D&D Beyond, but depending upon your actual intended use it might suit?