It's been a while, mine are grown up now and it feels a bit to me as though children didn't quite do so many activities at such a young age as is normal now. For me swimming was the only absolute must as for me that's a safety thing (lessons only until they were competent swimmers, could do a length of the pool without touching the bottom etc, then we just took them swimming at the local pool casually). Other than that it was just dependent on their interests and what we could afford at the time, one child was into horses (my hobby also) so that did take up a lot of time and money so I wouldn't recommend lol! One turned out to be quite musical so we started her off with taster lessons on a variety of instruments until she found the one she liked best then did regular private music lessons on that from 7ish onwards, plus saturday music school/band from 11ish onwards I think, from that she asked to start a second instrument so did some extra classes for that, at secondary school she joined school choir and also did music theory and joined extra ensemble and performance groups etc (her school was quite musical luckily so lots of activities arranged via school). My youngest boy was more sporty but also not really a 'sticker' to any one thing so we did a lot of chopping and changing between different activities which he'd do for a term or two at a time before getting bored and whingy and asking to change to something else, think we went through football, touch rugby, cricket, tennis, scouts, karate, cadets, cycling proficiency, drama/stagecoach etc over the years, he didn't get to a brilliantly high standard in any but all good general experience and confidence/fitness building etc., he's more of an 'all rounder' than a specialist!
Where to start, you know your kids best, what kind of thing would they like, something sporty or more artistic, more outdoorsy or indoors, a group activity or more individual, then google what's available in your area and that fits your budget and schedule. Ideally I'd look at something that will let you do taster sessions or pay as you go rather than having to commit to a terms worth of lessons at once or commit to buying lots of expensive kit or equipment when you don't know if it's something they'll enjoy yet. Maybe look at brownies or cubs as a good all-round start as they do all sorts of different activities as well as being community minded which is a nice starting point? But generally I'd say don't feel too much pressure to fill their whole schedule with loads and loads of formal/organised extra curriculars at this young age, the main thing I'd say when they're this little is that they have lots of time for play and to use their imaginations, you can encourage this through doing crafts etc with them at home, encourage them to develop a love of reading (through library books etc), play music and dance together, take them out locally to parks or the countryside etc at weekends and holidays and so on. There's lots of time for their talents and interests to emerge as they get a bit older....