Thank you for all yours response. I add this topic to forum, because taking kids photos is crucial for a lot of photographers. But I'm skip introduction and go on further.
As I start with Nikon F65 and has analog Pentax in hands I can only say that old manul body without digital feautures has a lot of adventages like split prism. Make photos sharp it was easier in manual mode for me than on newer cameras. I skip here subject change focusing screen, because you can not to it on all new bodies like K1 series. It is guarantee issue too. You can lost it after that.
For start I has only two lens 100 WR new Pentax and 77 limited for working with kids. It is not business, but more family documentation time. I like taking photos. I do not make it for living and I can spend more time to learn new techniques. SDM lens is good idea, but now I am out of budget. From other hand I am looking for the lightest lens, because when my kids will grow up I want go with them to mountains. At this situation all grams matters. My target is get the smallest possible number of lens and make them universal fx. use one len as tele on APS-C and portrait on FF body. I want the most specific job - get all stuff which I have on journey and taking photos without pain in my back. I hate on this scenario change lens too often. I tried it with old manual lens fx. 28mm, 50mm in mountain and it was nightmare. I spend more time with changing lens than taking shots.
After 2 thousand pictures with AFC I can say that AFS is better on K3 II for small children in dark rooms. It easier to get focus. I use SEL (one point only focus), but the most time I take photos with the most demanding situation - very low light and F1.8, F2. At this situation any mistake in focus and picture can be lost. On daylight I do not have a lot of troubles with 77. I start feeling it. Except small detail I'm not comfortable with my macro 100mm len. I am learning how use in out macro scenario.
My current plan is use 77 limited on K1 II and 100mm macro as backup fx. more distance shoots on APS-C. It is crazy idea, but I would try use them together at the same time (of course with changing body with lens for shots).
On my previous post I wrote about unfocus when I take photos with AF point SEL (one only) selected and targeted on eye and when light is low (it is my specific situation - when I go to home is night). From medic reason I can not use full light when I take shots my newbord kids, because of their specific health condition. In the future I expect that they will be more mobile and it is why I ask you dear Pentaxian!
At my 5c I think marketing is huge reason why people changing bodies. I know people which take photos in masses and get another system because they want new lens, AF and some fancy stuff. As I wrote before. It is really strange that bodies I splitted on categories for sport, landscape etc., because the most of them minus Live view modes like eye focusing is still old fashion design without nothing spectacular. Difference I see in buttons and sensors, but outside that I can not find any spectacular change. Using DSLR in Live mode it is outside specific situation when fx. you do not want lay down is like back to compact camera for me. Maybe I am old fashion by I am looking for in the future bodies with manual working comfort like in old film cameras without film, but with its dynamics and possibilities.
OK. I ended offtopic here.
After my two initial question how succefully take photos of kids and is it possible I will be add one to:
3. How 100 macro and 77 limited has adventage or disadventage on the field taking kids photo? Have you any experience?
My observations:
77 limited - not very good for small space in room and when you want document few people around kids on APS-C body (K3 II). It is too narrow. In low light in AFC likes missing. ISO above 3200 limited IQ on eyes and it is what you can see. It is good lens, but outside or in large space behind you. In other place you have to use very strange angles to get frame. It is more selectable in composition. Maybe troublesome in small space when kids crawling to you.
100 macro - good for detail. It is a pleasure take face with toys in small space. It is not limitation, but it is very narrow degree in composition. It is hard to use it for kids in home with another person. Without large space behind is that not possible. It is great choice for smile or eye details.