I have a bigger age gap this time around and my 5 year old guessed even before I did the preg test, and I have always been very determined not to lie to her (presenting things in a child friendly way but not lying) so I told her that one of the eggs in my tummy might be growing into a baby but that we couldn't be sure til the doctor had checked - and confirmed it after the 2 week scan to both her and DS (3).
DS was born the day after DD's 2nd birthday though and I told DD really early on, and the children I child minded then who were also all under 2 when I found out I was pregnant - I took them all to my midwife apts (I did get some looks in the waiting room with a bump and 3 little girls under 2
but they were all very good and interested in hearing the heart beat etc.)
I think the only reason not to say is if you are worried about loss, but I think children understand and can accept with more than we give them credit for as long as we present it thoughtfully - DD was interested in and talked about (and to) baby right through my pregnancy when she was nearly 2 and really did seem to have a good understanding (on her own level). We looked at pictures of babies at different stages and talked about how big he was and how big he needed to be to come out "safely" and this kept it "real" to her - but I agree don't go on and on about it once you've told the DC, just let them take the lead with questions.
This time around both DC seem to understand well at their own levels, although DD is far more interested and excited and DS sometimes very proud and boasting to his friends or talking to the baby through my tummy, other times he wants to pretend to be my baby - normal reactions and fine, as I have explained I can't carry him much atm but will be able to again when my tummy has mended after baby comes out (know I will be having a planned section) - I think it is important to set expectations and for him to know it is not the end of me carrying him, I will be able to again at a fixed point in future (he's a big lad though, not carryable that far even without being preg or c-section wound!). It has helped this time to look at pics of babies growing in the womb, to compare baby now and baby at the right size to come out to size of dolls and teddies, and also handily as my c-section is 2 days after Easter it is handy to explain the baby should be here just after the Easter bunny!
If a toddler is oblivious and you are not ill or struggling (or being kicked or climbed on around the tummy by toddler) I guess no hurry to tell them, but if they have guessed I do think it is a little odd to choose to lie about the baby, esp after the 12 week scan!
All the best!