My DD is also very tall/slim & I'm an experienced pattern cutter, so can help, but I'm wondering is the 18 month version measurements taken from you measuring the actual pattern, or what the back of your pattern says is the 18mnth body measurements??, as even though I'm used to it with my own DD, a 4 yr old fitting an 18moth width sounds quite extreme??
Its slightly more complicated than you ask, but not too much, so that would pretty much work, especially if its a Skirt????....a dress would require allowing for DDs neck & armholes not being as small as her width IYSWIM
For a SKIRT, using the difference of the Pattern CHART size measurements NOT Pattern
eg DDs size on Pattern Chart says fits waist 50cm & your DD measures 44cm, you have 2 pattern panels, 1 back & 1 front.....you need to reduce it by 6cm in total, so 3cm on each pattern piece :)
SO, if hips are the same difference or similar, simply fold your pattern peices & draw a line 1.5cm (half of 3cm) from the folded edge......pin this, or lay it along the edge of the FOLDED fabric
hope that makes sense