I have been minding for years, have had three children of my own (including twins) and have got a wealth of experience of every age group, and probably every mix of ages you can think of too. I am not a "babysitter"; I do a professional job, albeit in my home, at caring for children. There are lots of similarly-minded childminders out there.
We can get support through local groups and networks, and can take advantage of training courses. In the same way that you, as a mum, care for two children of different ages, so can childminders.
When out walking, I use buggies (I have a choice of a single, double or triple), and always use reins or wrist-straps on young children. I think that most childminders are quite hot on safety issues. To be honest, if I had a mix of children that I would struggle to "contain" in a park or indoor soft play, then I wouldn't go that day.
You have already had a childminder, so you probably know what you are looking for in a new one. When you visit, if you ask the same questions as you have here, and you don't like the answers, then you probably won't choose that childminder. If you don't have confidence in the minder's abilities then, again, you aren't going to choose that childminder.