Set iron-ajax parameter from a shadow root - ajax

Question

I would like to create an element which changes the parameters of an iron-ajax element located in another root (its parent)
Parent:
<iron-ajax id="ajaxService"
auto
url="http://..."
method="GET"
handle-as="json"
params="{{ajaxParams}}"
last-response="{{datas}}"></iron-ajax>
<template is="dom-repeat" items="[[datas]]">
<div>[[item]]</div>
</template>
and the child:
<dom-module id="my-pagination">
<template>
<style include="shared-styles">
:host {
display: block;
}
.disabledButton
{
color: #DDDDDD;
}
</style>
<div class="horizontal center layout">
<div class="flex"></div>
<span>
<span id="rangeStart"></span>-<span id="rangeEnd"></span> sur <span>[[data.total_count]]</span>
</span>
<paper-icon-button id="prevPage" class="disabledButton" icon="mdi:chevron-left" on-tap="prevPage" disabled></paper-icon-button>
<paper-icon-button id="nextPage" icon="mdi:chevron-right" on-tap="nextPage"></paper-icon-button>
</div>
Polymer({
is: 'my-pagination',
properties: {
ajaxId: String,
offset: {
type: Number,
value: 0
},
limit: {
type: Number,
value: 6
},
usersAjaxParams: {
type: String,
computed: "processParams(offset, limit)"
}
},
processParams: function (offset, limit) {
return {
offset: offset,
limit: limit,
};
},
computeRange: function (offset, limit, nodeRangeStart, nodeRangeEnd) {
nodeRangeStart.innerText = offset;
nodeRangeEnd.innerText = offset + limit;
},
prevPage: function () {
this.offset = this.offset - this.limit;
this.computeRange(this.offset, this.limit, this.$.rangeStart, this.$.rangeEnd);
};
},
nextPage: function () {
// pretty much same code as above, but for the "next" page
}
},
attached: function () {
this.computeRange(this.offset, this.limit, this.$.rangeStart, this.$.rangeEnd);
}
});
So what I am trying to achieve here is to create a '' element which i could use by providing the ajax node ID as property, and the pagination element would change the ajax parameters whenever I click the "next" or "previous" page button.
And while we're at it, how I can pass datas to the child as weel? (I will need '[[data.total_count]]' later on)
Thanks in advance!!