OP you are being vvvvv unreasonable and legally don't have any basis to fight this - if you've seen the will and confirmed it doesn't name your conceived but unborn child, and doesn't include your jurisdiction's wording about "conceived but unborn" children, you just don't have any basis to "fight" it.
As other posters have mentioned, at least in the English & Welsh legal system, you cannot (the executor cannot) amend distribution of the estate even if all minors and their legal guardians agree to it - it cannot be done.
Your father should have had this explained to him as part of making the will.
Which brings me onto the point that a few other posters seem to be blatantly ignoring - get rid of any notion of the BIL doing what someone considers "the decent thing" here. There is no "decent" wiggle room because the Executors role is to carry out the written wishes of the deceased, and distribute it accordingly. There's no area for personal interpretation or "oh FiL wrote this but he really meant that other thing"!
It is NOT up to an Executor to veer from the legally valid wishes if the deceased - if they do, they are (quite rightly) opening themselves up to legal challenge after the fact. That's why wills are so important - they MUST be executed properly, with a few options around varying the distribution in a few select legal cases which needs more than a layman's i-think-i-know-best-or-kinder idiocy.
Your BIL could be challenged on distribution to the adopted grandchild, that has legal standing if the adoption is above board (in fact you may wish to mention he should take legal advice on this if you're being kind to him so as to not end up in hot water later on) but put all notions about your child inheriting here... It's madness and has no legal claim.