My daughter loves cellphones. Whenever I show her my phone, she starts moving her mouth like she's talking, but she doesn't seem to know what to say.
この場合、次のような言い方ができますよ。
ーMy daughter loves cellphones. Whenever I show her my phone, she starts moving her mouth like she's talking, but she doesn't seem to know what to say.
「娘は携帯が大好きで、私の携帯を見せるといつもお話をするように口を動かしますが、何を言っていいかわからないようです」
to start moving her mouth like she's talking で「話をしているように口を動かし始める」
doesn't seem to know what to say で「何を言っていいかわからないようだ」
ご参考まで!