Ela não fez isso!

A criança é sua filha adotada .

^{Sem vergonha copiada da resposta MY OWN em SF & F}

Não há indicação específica de que Maddie é a filha de Clarke (exceto o envelhecimento avançado), já que ela tem claramente mais de 6 anos de idade (menos tempo de gravidez).

Jason Rothenburg, produtor de The 100 ...

We wanted a reveal that Clarke survived and that her Nightblood kicked in, and she's now essentially — or so we think for the first couple of minutes of that final pop-out at the end — the lone survivor on the planet. Her friends... we know they made it to space, but she doesn't know whether or not they made it. We also know there's a group of 1200 led by Octavia (Marie Avgeropoulos) in the bunker, but Clarke is the lone survivor on the planet. And then we reveal she's found Maddy, this Nightblood child. Clearly, there's a maternal connection between the two of them when we reveal her in the end.

...

They have been together for years when we first see them. She's been with Maddy alone for a number of years; longer than she was ever on the ground with anybody. So it's going to be a very powerful bond and a huge part of season five.

^{Hollywood Reporter}

Encontrado não indica uma filha, pelo menos, não uma filha biológica.

De fato ... em outra entrevista ... Rothenberg confirma

The mother-daughter vibe that you pick up on in that scene is real. (It’s not her biological child; I think it’s fairly obvious that the kid is too old for that.) But she is a nightblood. Clarke was alone until she found Madi. When we find them together in [episode] 413, they have been together already for a number of years and are relying on each other and surviving together.

^EW.com

Quanto a quem Maddy / Madi (a grafia varia) é e de onde ela vem ainda está para ser revelado em S5.