Underweight. Much concerned about this.

Hello,my daughter is 20 months old. She is very active but is underweight. She didnt have motherfeed because her mother not able to generate. When she is one month old she was diagnosed with mild hydronephrosys,for which we had taken treatment from pgi chandigarh. We gave her amoxycillin which we had withdraw recently only. We didnt able to go to routine chkup due to covid. Her las report shown gud improvement. She didnot eat her meals well as her mother usually take an hour to complete her meal. Although we gave her quite healthy things like oats, daliya, halwa, fruits, healthy vegetavles etc. My daughter drinks plenty of water in whole day too.. My concern is why she is so much under weight compared to children of her ages.her height is growing decently well. her hb is around 11. Doctor has recommended us to give her 5ml iron twice daily. She is currently weighs 8 kg only and we are very much concerned about this.