That the children should be able, etc. - שיהו התינוקות כולי
According to ר"מ: a) Playing with children is more docile than not goring people, b) not goring cows for three days is certainly sufficient, and c) three times not goring cows in one day is insufficient.
And he will not guard him now; he is liable - ולא ישמרנו האידנא חייב
רש"י (initially) maintains that according to רבא a מועד pays a נ"ש by the third נגיחה. However תוספות maintains that a מועד pays a נ"ש only by the fourth נגיחה. According to אביי the fourth נגיחה must take place on the fourth day while according to רבא it may take place even on the third day. There is a dispute whether if a שור הנסקל שנגמר דינו is אסור בהנאה or מותר בהנאה . Three sets of עדים זוממין regarding the goring of an ox to make him a מועד, may have to pay for the fourth נגיחה , or possibly they only pay for his devaluation but not for future damages.