I would guess it is some kind of expansion joint. To allow for expansion and contraction of subfloor.

how old is the house as you may need to check for asbestos on walls and floor and is teh whole house a slab on ground if not the floor under the tiles may not be concrete..., when replacing floor tiles i would also replace skirts to ensure waterproofing is above the floor all around (and avoid a potential leak at the join) and depending on what you are planning to do (ëg install new shower stall or wet floor?) i would also consult your tiler about "levelling the floor" which is much more complicated than we think.. and take your time removing the old tiles, especially on the walls as it's better to do as little damage (to the structure and yourself) as possible and you will find many online DIY instructions for retiling that are worth reading before you start...good luck with your project

The first question -- you are asking about the bathroom ? Not the ensuite ? Just clarifying . And if it is the bathroom , what colour are the hall carpets ? If it was 'near' the lounge , looking at the laminate flooring , I'd prefer lighter flooring in the bathroom too , but right now I'm assuming it is the other end of the unit , and so I'd like to know about the immediate rooms surrounding it .

1200 x 900 is a generous shower for a kids bathroom (1200 x 2350 is GIANT sized for a luxury master suite) and the leftover space provides a dry floor area to stand and towel off as well as wall space for towel rails ... there should be plenty of space for a built in bath and shower with easy access from inside the shower to reach a towel and why not include a generous linen cupboard as well