The timing of the head shaving can be confusing. I believe most think it happened shortly after the children recovered from the measles. But as more is discovered, it seems their heads were shaved some time after that. After it became obvious their hair was coming out in clumps. So it was done in resignation to that fact, rather than done earlier in anticipation of it. What is also interesting is they are wearing sweaters in the resting photos , but are in blouses in the bald photo...which also suggests a later, warmer time
I found the exact date just this week in Nicholas & Alexandra's diaries. Here's what I posted Wednesday in a measles thread:
Alexandra Feodorovna, 3/16 June
Всех четверых девочек обрили наголо, так как после кори стали сильно выпадать волосы.
(All four girls were shaved bald, because after measles much of their hair fell out.)
Nicholas II, 3/16 June
Все четыре дочери сбрили себе волосы, так как после кори они сильно лезли.
(All four daughters shaved their hair, because after measles it fell out badly.)
And on the following day, Alexandra notes: "Aleksei had his hair also complete shaved off."