<select class="form-select form-select-solid mb-2" name="regroupements" data-control="select2" data-placeholder="Select something" data-allow-clear="true" multiple="multiple">

                        <option></option>

                        {% for niv1 in niveau1 %}

                              <option value="{{ niv1.id }}">{{ niv1.caption }}</option>

                                    {% for niv2 in niveau2 %}

                                          <option class="l2" value="{{ niv2.id }}" >{{ niv2.caption }}</option>

                                    {% endfor %}

                       {% endfor %}

                    </select>

.l2 {

    padding-left:20px !important;

  }