Why are there plenty of pictures of children with tattoos? They were selected for work too?
1. Non-Jewish children sent to Auschwitz were also tattooed. Gypsies, Poles and Russians weren't selected in the same way as Jews. Most Gypsy children were dead by the time of liberation but there were plenty of Poles and Russians left.
2. Jewish children sent on transports ending up in the Theresienstadt Family Camp were not selected and were registered and tattooed as part of a deception measure (a Potemkin camp inside Birkenau from which Jews could correspond under controlled conditions, creating the illusion of normality). They were later on gassed once the deception had played out, so they wouldn't be in any photos after liberation. Some teenagers in this cohort survived partially because an SS man who was otherwise a bit of a hard case ensured they were spared. Gosh, who'd have thought that individual behaviour would influence things...
3. It doesn't seem that long ago that we were discussing Mengele's experiments. Most of the Jewish children tattooed who might have survived to be photographed on liberation (bearing in mind the famous photos would have included Poles, Russians and others as well) belonged to this group.
4. From spring 1944, working age was lowered to 10-12 years old in the 'east' and within Auschwitz; this meant that children who appeared to be older could slip through whereas previously they couldn't. And the sheer numbers meant there were other opportunities to slip through. There was a children's block set aside to 'round up' such cases, and this block was subjected to repeated selections to weed out the smaller children, including on one occasion, a selection carried out by measuring height - anyone under a certain height went to the gas chambers.
5. Some of the final Jewish transports from Slovakia arrived after selections were stopped at the start of November 1944. Any Jewish children on them would have been registered and tattooed because Himmler had ordered a halt to the Final Solution.
Aside from lengthy chapters in Auschwitz 1940-1945 by Irena Strzelecka (who also wrote lengthy articles on Mengele in Hefte von Auschwitz), there's a lengthy section of the following book on the subject:
Buser, Verena, Ueberleben von Kindern und Jugendlichen in den Konzentrationslagern Sachsenhausen, Auschwitz und Bergen-Belsen. Berlin: Metropol, 2011
So much for the 'gotcha'.