SELECT 
  category_id, 
  parent_id 
FROM 
  cscart_categories 
WHERE 
  parent_id IN(
    3561, 3562, 3563, 3564, 3565, 3566, 3567, 
    3568, 3569, 3570, 3571, 3572, 3573, 
    3574, 3575, 3576, 3577, 3578, 3579, 
    3580, 3581, 3582, 3583, 3584, 3585, 
    3586, 3587, 3588, 3589, 3590, 3591, 
    3592, 3593, 3594, 3595, 3596, 3597, 
    3598, 3599, 3600, 3601, 3602, 3603, 
    3604, 3605, 3606, 3607, 3608, 3609, 
    3610, 3611, 3612, 3613, 3614, 3615, 
    3616, 3617, 3618, 3619, 3620, 3621, 
    3622, 3623, 3624, 3625, 3626, 3627, 
    3628, 3629, 3630, 3631
  )

Query time 0.00033

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_categories",
      "access_type": "range",
      "possible_keys": ["parent"],
      "key": "parent",
      "key_length": "3",
      "used_key_parts": ["parent_id"],
      "rows": 71,
      "filtered": 100,
      "index_condition": "cscart_categories.parent_id in (3561,3562,3563,3564,3565,3566,3567,3568,3569,3570,3571,3572,3573,3574,3575,3576,3577,3578,3579,3580,3581,3582,3583,3584,3585,3586,3587,3588,3589,3590,3591,3592,3593,3594,3595,3596,3597,3598,3599,3600,3601,3602,3603,3604,3605,3606,3607,3608,3609,3610,3611,3612,3613,3614,3615,3616,3617,3618,3619,3620,3621,3622,3623,3624,3625,3626,3627,3628,3629,3630,3631)"
    }
  }
}