You can keep your response straightforward when it comes to your ability to perform the work. If you do have restrictions when it comes to lifting, standing, bending, or more you should disclose this to the interviewer. Although legally, the interviewer may not be able to ask about physical hindrances or disabilities, it's in your best interest to be straightforward.
"From reading the job description I believe that I can perform the physical activities in this role. Nothing is restricting me from playing with the kids, lifting them, or standing most of the day. I am sure to wear practical clothing and comfortable footwear so that I can keep up with the little ones all day!"
"The only restriction that I have is my ability to lift more than 50 pounds. Seeing as most of the children in this age group will be under 25 pounds, I do not see any issues with my ability to perform the work as described in your job posting. I am active, fit, and energetic."