Whilst the baby dresses up to 24 months are of average sizing for the typical UK child, the older dresses (2-12 years) are quite slim fitting and dresses are worn much shorter in Spain than in the UK so unless your child is slimmer than average, you may wish to consider going up a size.

The official size guide is as follows (but dresses are short and narrow):

 

Size	For children of a height between	Chest sizes (of dresses, not child so please allow 1-2 cms for comfort) Smallest measurement is for fitted bodice styles ; largest size is for wider style dresses (in inches).
0-3m	56-62 cms
3-6m	62-68 cms	17-19"
6-12m	69-80 cms	19"
12-18m	81-86 cms	20"
18-24m	87-92 cms Please note that although the height is the same as the 2y this is cut for babies	20.5"
2 yrs	87-92 cms Please note that although the height is the same as the 24m this is cut for a small child rather than a baby shape	19-21"
3 yrs	93-98 cms	20.5-22"
4 yrs	99-104 cms	22-22.5"
5 yrs	105-110 cms	23-24"
6 yrs	111-116 cms	22.5-26"
7 yrs	117-122 cms	23.5-26"
8 yrs	123-128 cms	26"
10 yrs	129-140 cms	29"
12 yrs	141-152 cms	32"