I have issue regarding my first Child. He has weak and feeble bones/body. Most are times he falls down while walking or carrying even lighter things his peers could bear. I need a prayer or anything I can use, for him to be strong enough.

This is the dua narrated from Imam Jaffer bin Muhammad As Sadiq (a.s) please recite 3 time this Dua and rub over his legs May Allah give him fast and complete recovery(Ameen)
اللَّهُمَّ إِنِّي أَسْأَلُكَ بِحَقِّ الْقُرْآنِ الْعَظِيمِ الَّذِي نَزَلَ بِهِ الرُّوحُ الْأَمِينُ وَ هُوَ عِنْدَكَ

فِي أُمِّ الْكِتَابِ عَلِيٌّ حَكِيمٌ أَنْ تَشْفِيَنِي بِشِفَائِكَ وَ تُدَاوِيَنِي بِدَوَائِكَ وَ تُعَافِيَنِي
مِنْ بَلَائِكَ.
Also if your child can recite please teach him otherwise you constantly read these line for him:

بسم الله الرحمن الرحیم یا منزل الشفا و یذهب الداء صل علی محمد و اله و انزل علی وجهی الشفاء

بسم الله الرحمن الرحیم یا هو یا من هو یا من لیس الا هو صل علی محمد و آل محمد
One small zikr always chant :
يا غفور
You may write above dua on a piece of paper as a amulets and put it in his arm.