You can see the teeth on both sides of his front teeth on the top.
The teeth on both sides of his top front teeth have started to come in.
この場合、次のような言い方ができますよ。
ーYou can see the teeth on both sides of his front teeth on the top.
「上の前歯の両側に歯を見ることができる」＝「上の前歯の両側に歯が見え始めた」
one's front teeth on the top で「上の前歯２本」
teeth は複数形なのでこう言うことで前歯２本を言えます。
ーThe teeth on both sides of his top front teeth have started to come in.
「上の前歯の両側の歯が生えてきた」
one's top front teeth で「上の前歯２本」とも言えます。
to start to come in で「生え始める」
ご参考まで！
カナダ 
