Is there.....

[Bryon_Soulweaver]

Is there any templates for a child of a demon lord?

 

[Pinotage]

Half-fiend could probably work, though that's not specific to a demon lord.

Pinotage

 

[Thia Halmades]

See Tiefling (Forgotten Realms) or Unearthed Arcana (d20). Either of those should fit your bill, although Tieflings are a racial packet, UA can give you a "monster level progression" or teach you how to do it with little pain.

Also, you may consider modifying Dragon Disciple and swapping out powers and playing with alignments to create a Demon prototype PrC.

 

[Merlion]

Half-Fiend would probably work better than Tiefling for the direct child of a demon lord. I'd take Half-Fiend and tweak it to make it more specific, and maybe a tad more powerful

 

[Bryon_Soulweaver]

OK, thanks for the advice. I was just wondering if there was one out there.