Some good advice mentioned above and I suggest you take it all on board. Clearly your ambition is to become a surgeon and you sound like you're pretty determined to do it, so stop thinking about doing GP just for the flexibility if it's not what you want to spend the rest of your career doing. Choosing which specialty to pursue is an important decision because you will be spending so long training in it - you want to make sure you're doing it for the right professional reasons and not what suits your family. First and foremost you should focus on getting into surgery - it's already competitive enough and might take you more than one round of applications. Then making sure you can do the job properly, making progress and that you are getting some satisfaction out of it, managing the workload, work-life balance, not getting burnt out etc. You might find all of these things more challenging when you have young children than you would if you delayed having children until later.
Having said that, you should realise that being a surgeon requires you to make sacrifices in your family life regardless of your gender. If being a young mum is more important to you then maybe it's a good idea to take a career break and/or go LTFT early on, but that will delay your training and becoming a consultant until later. I would argue that it's better to have children later on when you do higher surgical training, or even when you become a consultant - by this time you won't be competing for jobs any more and perhaps moving around much less than you will be in CST. I've talked to a few people in similar situations to yours and I always say don't make so many rigid plans, like I must become a mum by whatever age. Life never goes exactly to plan, we have setbacks to overcome along the way, priorities change, people change.
Also, something that nobody has mentioned so far: do you have a man? Doesn't he have an opinion about this? Some men are perhaps not as supportive of women becoming surgeons when they realise how much dedication of time and energy it needs, particularly if they aren't also doctors. Have you discussed this with your man?