Is there an error in a children's toy that recites Surat Al-Fatiha and then follows it with "صدق الله العظيم" (Allah the Almighty has spoken the truth) after "آمين" (Amen)?

The word "Amen" is not part of Al-Fatiha; rather, it is a recommended affirmation for the supplication contained within the surah. As for saying "صدق الله العظيم" (Allah the Almighty has spoken the truth) after recitation, it is not required. There is no harm in buying the toy, but children should be alerted that what comes after the end of Al-Fatiha is not part of it, and they will distinguish Al-Fatiha from other things when they grow up.