I often shadow a young boy with special needs at my local shul on Shabbat. This boy does not stay still and moves quickly and goes in and out of shul, where his father is davening. Although I daven earlier, I have the problem of not always being able to stand still during Kedusha, as I have to run after him so he does not get hurt, etc. Recently, I was scolded by an older man for this. Despite my explanation of the situation, he said that one must stay still during Kedusha even in the face of mortal danger. What does Halacha have to say about this situation? Answer:
Most of the discussion about walking is found in regard to Shemoneh Esrei, and we will start with that.
309 - Ushering in an Avel after Sunset of Shabbat
310 - Chasing after a Child during Kedusha
Your shul-mate may be remembering (incorrectly) the following mishna (Berachot 30b). During Shemoneh Esrei, "even if a snake is wrapped around his leg (parallel to war), he should not stop." However, the gemara (ad loc. 33a) says that this is only when it does not appear that the snake presents real danger. Furthermore, the stop (hefsek) referred to is speaking, e.g., calling someone to save him. One actually is allowed to walk to another place to protect himself from even a moderately precarious situation, as walking is not a real hefsek (Mishna Berura 104:10).
Of course, one should not walk for no good reason during Shemoneh Esrei, as it is a low-level hefsek. We find the following priority list for one who must take care of something during Shemoneh Esrei (Mishna Berura 104:1), from best to worst: 1) hinting to someone to help without talking or moving; 2) walking somewhere without speaking; 3) speaking, which is permitted under only extraordinary circumstances.
In comparison to Shemoneh Esrei, the position of one’s body is less important during Kedusha. The Shulchan Aruch (Orach Chayim 95:1) says one is required to have his feet together during Shemoneh Esrei, to "imitate" angels in service of Hashem. In contrast, regarding Kedusha he writes (ibid. 4) that it is good (i.e., less critical than for Shemoneh Esrei) for one’s feet to be together. Again, one does not walk for no reason, but legitimate concern for a special needs child’s physical (or emotional) safety is fully justified. The additional liturgy for Shabbat within Kedusha is not even considered a full-fledged part of Kedusha (see Mishna Berura 125:1) and arguably it is even less problematic to walk then. Furthermore, one who is not davening has a lower-level obligation to join Kedusha, to not look like he does not endorse what is being said. This would seem to not apply to one whose pressing preoccupation is clear to all. So halachically, you are clearly fine.
We continue with a conjecture about your shul-mate’s reaction. Although he said he was motivated by concern for your Kedusha obligations, it is likely that he was bothered by something else. Having a child running wildly through a shul with or without an adult chasing after him is not ideal for the atmosphere of a shul or davening. Those with certain personality types are particularly disturbed by such a situation. Some people properly care greatly about decorum and quiet in shul. The situation may make others just feel nervous. You may not think in such terms, because you are, laudably, concerned with the welfare and happiness of the child.
It can be a good idea to discuss such a situation (presumably, the father) with the rabbi or other leadership. The job of any true leader is to strike a proper balance between the needs of the general community and the unique needs of individuals. We cannot be of help from here, as only someone intimately familiar with the setting and the people involved can do it justice. Obviously, no reasonable rabbis would ignore the needs of a special needs child. However, it is plausible to arrive at an arrangement using discretion as to when the child will spend time in shul. But again, the feelings of the man you refer to (even though he is wrong in what he said and seemingly how he said it) and perhaps other people is a possible issue, not the halachot of Kedusha.