Please don't worry - you're SO young, I promise! You have time. I did it the other way round, foot down on my career through my 20s and early 30s and am now trying to start a family in my mid 30s. The grass is always greener, I'm sure, but I've often thought there's a pretty good argument for getting through the kid years early if you know you've met the right person. Neither route is "right" but I've always suspected that doing it either fairly early or fairly late probably does the least damage to careers.
If I were you, I'd make a plan for the next couple of years to make sure that you have the best possible chance of getting onto the course you want. If speech & language therapy is your first choice, then go for it - don't compromise. I'd email the course leaders at a few different unis and ask advice on the entry requirements / the most useful form of voluntary experience. I'd also try to speak to a range of practicing speech therapists to see what they advise. I bet people will be more than happy to have a coffee and chat about routes in.
I think you mentioned that your Mum could do a bit of child care? If she could take the kids one day per week - or even once a fortnight - then if you could find relevant voluntary experience on that day, it will really add up over two years. You could also try to get hold of the first year reading list for your ideal course and start doing some of that reading now?
Whether or not 'mum skills' help you to get onto a course, I bet they will massively kick in once you're actually in the work place - you will probably find many things a LOT easier than fresh graduates! You will initially feel way behind the career curve, I imagine, but then when the fresh grads alongside you reach their later 20s / 30s and start taking time out for family, your kids will be much older and you will be able to keep your foot down. It'll all even out in the end.
Stick by your decision. It definitely has its advantages - so make the most of them!
Good luck!