A standard way of doing this is to use census data. I got the census data for Pakistan from United States Census Bureau’s International Data Base.
https://www.census.gov/programs-surveys/international-programs/data/tools.html
Choose the International Database (IDB) tool, Select Pakistan, then Population by Age. I found:
Age Population
0 6156303
1 6025699
2 5964082
3 5916546
4 5867268
5 5819072
6 5762670
7 5729713
8 5708345
9 5652476
10 5563202
11 5471471
12 5382154
13 5238823
If we add these up we get:
80308125 for ages 0 thru 13
and:
24062930 for ages 0 thru 3
That is about 29.96% of childen aged 0 thru 13 are aged 0 thru 3.
We can apply this proportion to your populations data giving:
Taluka/District Age 0-14 Age 0-3
------------------- ------------- -----------
Daharki 145132 43482
Ghotki 236860 70963
Khan Garh 69909 20945
Mirpur Mathelo 153125 45876
Ubauro 165084 49459
------------------- ------------- -----------
District Ghotki 770112 230726
I hope this is of some use.
You should check my arithmetic before using any of the results presented in this post.